I’ll break it down for you.
sudo apt-get update
sudo is a program (most of what you’ll care or interact with live under /usr/bin). In layman terms what this program does is prompt you for your password so you can perform whatever else after it as an administrator.
apt-get is the default package manager for Debian class distributions. Think App Store.
update is a command line parameter for the apt-get package manager. As the name implies this performs an update of all your systems packages.
So what the above command essentially does is to: as an admin, update the local package cache to any newer package provided by the configured repositories.
The next command typically done after that is to run sudo apt-get upgrade to perform the actual upgrade of the packages that it downloaded earlier.
Note: on Debian this is a two step process. Most other distros just have the one update function which performs both the update and upgrade of packages.
Filesystem Hierarchy
HIER(7) Linux Programmer's Manual HIER(7)
NAME
hier - description of the filesystem hierarchy
DESCRIPTION
A typical Linux system has, among others, the following directories:
/ This is the root directory. This is where the whole tree starts.
/bin This directory contains executable programs which are needed in single user mode and to
bring the system up or repair it.
/boot Contains static files for the boot loader. This directory holds only the files which are
needed during the boot process. The map installer and configuration files should go to
/sbin and /etc. The operating system kernel (initrd for example) must be located in
either / or /boot.
/dev Special or device files, which refer to physical devices. See mknod(1).
/etc Contains configuration files which are local to the machine. Some larger software pack‐
ages, like X11, can have their own subdirectories below /etc. Site-wide configuration
files may be placed here or in /usr/etc. Nevertheless, programs should always look for
these files in /etc and you may have links for these files to /usr/etc.
/etc/opt
Host-specific configuration files for add-on applications installed in /opt.
/etc/sgml
This directory contains the configuration files for SGML (optional).
/etc/skel
When a new user account is created, files from this directory are usually copied into the
user's home directory.
/etc/X11
Configuration files for the X11 window system (optional).
/etc/xml
This directory contains the configuration files for XML (optional).
/home On machines with home directories for users, these are usually beneath this directory,
directly or not. The structure of this directory depends on local administration deci‐
sions (optional).
/lib This directory should hold those shared libraries that are necessary to boot the system
and to run the commands in the root filesystem.
/lib<qual>
These directories are variants of /lib on system which support more than one binary format
requiring separate libraries (optional).
/lib/modules
Loadable kernel modules (optional).
/lost+found
This directory contains items lost in the filesystem. These items are usually chunks of
files mangled as a consequence of a faulty disk or a system crash.
/media This directory contains mount points for removable media such as CD and DVD disks or USB
sticks. On systems where more than one device exists for mounting a certain type of
media, mount directories can be created by appending a digit to the name of those avail‐
able above starting with '0', but the unqualified name must also exist.
/media/floppy[1-9]
Floppy drive (optional).
/media/cdrom[1-9]
CD-ROM drive (optional).
/media/cdrecorder[1-9]
CD writer (optional).
/media/zip[1-9]
Zip drive (optional).
/media/usb[1-9]
USB drive (optional).
/mnt This directory is a mount point for a temporarily mounted filesystem. In some distribu‐
tions, /mnt contains subdirectories intended to be used as mount points for several tempo‐
rary filesystems.
/opt This directory should contain add-on packages that contain static files.
/proc This is a mount point for the proc filesystem, which provides information about running
processes and the kernel. This pseudo-filesystem is described in more detail in proc(5).
/root This directory is usually the home directory for the root user (optional).
/sbin Like /bin, this directory holds commands needed to boot the system, but which are usually
not executed by normal users.
/srv This directory contains site-specific data that is served by this system.
/sys This is a mount point for the sysfs filesystem, which provides information about the ker‐
nel like /proc, but better structured, following the formalism of kobject infrastructure.
/tmp This directory contains temporary files which may be deleted with no notice, such as by a
regular job or at system boot up.
/usr This directory is usually mounted from a separate partition. It should hold only share‐
able, read-only data, so that it can be mounted by various machines running Linux.
/usr/X11R6
The X-Window system, version 11 release 6 (optional).
/usr/X11R6/bin
Binaries which belong to the X-Window system; often, there is a symbolic link from the
more traditional /usr/bin/X11 to here.
/usr/X11R6/lib
Data files associated with the X-Window system.
/usr/X11R6/lib/X11
These contain miscellaneous files needed to run X; Often, there is a symbolic link from
/usr/lib/X11 to this directory.
/usr/X11R6/include/X11
Contains include files needed for compiling programs using the X11 window system. Often,
there is a symbolic link from /usr/include/X11 to this directory.
/usr/bin
This is the primary directory for executable programs. Most programs executed by normal
users which are not needed for booting or for repairing the system and which are not
installed locally should be placed in this directory.
/usr/bin/mh
Commands for the MH mail handling system (optional).
/usr/bin/X11
is the traditional place to look for X11 executables; on Linux, it usually is a symbolic
link to /usr/X11R6/bin.
/usr/dict
Replaced by /usr/share/dict.
/usr/doc
Replaced by /usr/share/doc.
/usr/etc
Site-wide configuration files to be shared between several machines may be stored in this
directory. However, commands should always reference those files using the /etc direc‐
tory. Links from files in /etc should point to the appropriate files in /usr/etc.
/usr/games
Binaries for games and educational programs (optional).
/usr/include
Include files for the C compiler.
/usr/include/bsd
BSD compatibility include files (optional).
/usr/include/X11
Include files for the C compiler and the X-Window system. This is usually a symbolic link
to /usr/X11R6/include/X11.
/usr/include/asm
Include files which declare some assembler functions. This used to be a symbolic link to
/usr/src/linux/include/asm.
/usr/include/linux
This contains information which may change from system release to system release and used
to be a symbolic link to /usr/src/linux/include/linux to get at operating-system-specific
information.
(Note that one should have include files there that work correctly with the current libc
and in user space. However, Linux kernel source is not designed to be used with user pro‐
grams and does not know anything about the libc you are using. It is very likely that
things will break if you let /usr/include/asm and /usr/include/linux point at a random
kernel tree. Debian systems don't do this and use headers from a known good kernel ver‐
sion, provided in the libc*-dev package.)
/usr/include/g++
Include files to use with the GNU C++ compiler.
/usr/lib
Object libraries, including dynamic libraries, plus some executables which usually are not
invoked directly. More complicated programs may have whole subdirectories there.
/usr/lib<qual>
These directories are variants of /usr/lib on system which support more than one binary
format requiring separate libraries, except that the symbolic link /usr/lib<qual>/X11 is
not required (optional).
/usr/lib/X11
The usual place for data files associated with X programs, and configuration files for the
X system itself. On Linux, it usually is a symbolic link to /usr/X11R6/lib/X11.
/usr/lib/gcc-lib
contains executables and include files for the GNU C compiler, gcc(1).
/usr/lib/groff
Files for the GNU groff document formatting system.
/usr/lib/uucp
Files for uucp(1).
/usr/local
This is where programs which are local to the site typically go.
/usr/local/bin
Binaries for programs local to the site.
/usr/local/doc
Local documentation.
/usr/local/etc
Configuration files associated with locally installed programs.
/usr/local/games
Binaries for locally installed games.
/usr/local/lib
Files associated with locally installed programs.
/usr/local/lib<qual>
These directories are variants of /usr/local/lib on system which support more than one
binary format requiring separate libraries (optional).
/usr/local/include
Header files for the local C compiler.
/usr/local/info
Info pages associated with locally installed programs.
/usr/local/man
Man pages associated with locally installed programs.
/usr/local/sbin
Locally installed programs for system administration.
/usr/local/share
Local application data that can be shared among different architectures of the same OS.
/usr/local/src
Source code for locally installed software.
/usr/man
Replaced by /usr/share/man.
/usr/sbin
This directory contains program binaries for system administration which are not essential
for the boot process, for mounting /usr, or for system repair.
/usr/share
This directory contains subdirectories with specific application data, that can be shared
among different architectures of the same OS. Often one finds stuff here that used to
live in /usr/doc or /usr/lib or /usr/man.
/usr/share/dict
Contains the word lists used by spell checkers (optional).
/usr/share/dict/words
List of English words (optional).
/usr/share/doc
Documentation about installed programs (optional).
/usr/share/games
Static data files for games in /usr/games (optional).
/usr/share/info
Info pages go here (optional).
/usr/share/locale
Locale information goes here (optional).
/usr/share/man
Manual pages go here in subdirectories according to the man page sections.
/usr/share/man/<locale>/man[1-9]
These directories contain manual pages for the specific locale in source code form. Sys‐
tems which use a unique language and code set for all manual pages may omit the <locale>
substring.
/usr/share/misc
Miscellaneous data that can be shared among different architectures of the same OS.
/usr/share/nls
The message catalogs for native language support go here (optional).
/usr/share/sgml
Files for SGML (optional).
/usr/share/sgml/docbook
DocBook DTD (optional).
/usr/share/sgml/tei
TEI DTD (optional).
/usr/share/sgml/html
HTML DTD (optional).
/usr/share/sgml/mathtml
MathML DTD (optional).
/usr/share/terminfo
The database for terminfo (optional).
/usr/share/tmac
Troff macros that are not distributed with groff (optional).
/usr/share/xml
Files for XML (optional).
/usr/share/xml/docbook
DocBook DTD (optional).
/usr/share/xml/xhtml
XHTML DTD (optional).
/usr/share/xml/mathml
MathML DTD (optional).
/usr/share/zoneinfo
Files for timezone information (optional).
/usr/src
Source files for different parts of the system, included with some packages for reference
purposes. Don't work here with your own projects, as files below /usr should be read-only
except when installing software (optional).
/usr/src/linux
This was the traditional place for the kernel source. Some distributions put here the
source for the default kernel they ship. You should probably use another directory when
building your own kernel.
/usr/tmp
Obsolete. This should be a link to /var/tmp. This link is present only for compatibility
reasons and shouldn't be used.
/var This directory contains files which may change in size, such as spool and log files.
/var/account
Process accounting logs (optional).
/var/adm
This directory is superseded by /var/log and should be a symbolic link to /var/log.
/var/backups
Reserved for historical reasons.
/var/cache
Data cached for programs.
/var/cache/fonts
Locally-generated fonts (optional).
/var/cache/man
Locally-formatted man pages (optional).
/var/cache/www
WWW proxy or cache data (optional).
/var/cache/<package>
Package specific cache data (optional).
/var/catman/cat[1-9] or /var/cache/man/cat[1-9]
These directories contain preformatted manual pages according to their man page section.
(The use of preformatted manual pages is deprecated.)
/var/crash
System crash dumps (optional).
/var/cron
Reserved for historical reasons.
/var/games
Variable game data (optional).
/var/lib
Variable state information for programs.
/var/lib/hwclock
State directory for hwclock (optional).
/var/lib/misc
Miscellaneous state data.
/var/lib/xdm
X display manager variable data (optional).
/var/lib/<editor>
Editor backup files and state (optional).
/var/lib/<name>
These directories must be used for all distribution packaging support.
/var/lib/<package>
State data for packages and subsystems (optional).
/var/lib/<pkgtool>
Packaging support files (optional).
/var/local
Variable data for /usr/local.
/var/lock
Lock files are placed in this directory. The naming convention for device lock files is
LCK..<device> where <device> is the device's name in the filesystem. The format used is
that of HDU UUCP lock files, that is, lock files contain a PID as a 10-byte ASCII decimal
number, followed by a newline character.
/var/log
Miscellaneous log files.
/var/opt
Variable data for /opt.
/var/mail
Users' mailboxes. Replaces /var/spool/mail.
/var/msgs
Reserved for historical reasons.
/var/preserve
Reserved for historical reasons.
/var/run
Run-time variable files, like files holding process identifiers (PIDs) and logged user
information (utmp). Files in this directory are usually cleared when the system boots.
/var/spool
Spooled (or queued) files for various programs.
/var/spool/at
Spooled jobs for at(1).
/var/spool/cron
Spooled jobs for cron(8).
/var/spool/lpd
Spooled files for printing (optional).
/var/spool/lpd/printer
Spools for a specific printer (optional).
/var/spool/mail
Replaced by /var/mail.
/var/spool/mqueue
Queued outgoing mail (optional).
/var/spool/news
Spool directory for news (optional).
/var/spool/rwho
Spooled files for rwhod(8) (optional).
/var/spool/smail
Spooled files for the smail(1) mail delivery program.
/var/spool/uucp
Spooled files for uucp(1) (optional).
/var/tmp
Like /tmp, this directory holds temporary files stored for an unspecified duration.
/var/yp
Database files for NIS, formerly known as the Sun Yellow Pages (YP).
