bracktrack has all the apps pre-installed to crack and test wep. Reaver tool is the tool you use to crack WPA with WPS enabled: http://code.google.com/p/reaver-wps/ Not sure if the new versions of backtrack have this tool pre-installed. WPA and WPA2 can't itself really be cracked, its usually these 'user friendly' add ons like WPS that make them valnerable. WPS has to be on by default to get the wifi standard stamp of approval hence its so easy to crack into a WPA enabled modem.
yeah, tons of tutorials on cracking WEP online. basicaly just capture IV's and then you can crack the wep with aircrack in a few minutes.
WPA / WPA2 is harder. most tutorials will only show you how to do it with dictionary attacks. the first thing you need to do is capture the wpa handshake. to do this, run airecap and then do a deauthentication attack against someone already on the network (or wait for a new user to get on) i believe the deauth attack is "aireplay -0 1 -a <mac address of router> -c <mac address of client>". now you have the handshake, and you can go back to your home or whatever, everything else is now offline. WPA is salted with the name of the ap (the essid) so, if you have something like "linksys" you can download a rainbow table. if you dont, you have to brute force it! so you have to use pyrit to generate the pass on the gpu, pipe that into cowpatty, then pipe that into aircrack.
hope you understand that all, its from memory, so i might have messed up on something. but yeah, thats the general process.
I got board and set up my old router with WPA2 and cracked it with reaver, took 7 hours. Using CUDA to brute force it may be a better option because reaver can create a DoS and some routers will lock you out if the wrong PIN is entered too many times