What is SLI and CrossFire?

I'm new to building computers etc. and I've heard of these two terms and have no idea what they mean. I have used google but the answers I got were to complicated, is there a simple way to explain? If not thanks for your time!

SLI and Crossfire are the Same thing (From the prospective of a PC enthusiast) Basically the Term SLI (from the Nvidia Side of the GPU market is the process of using 2 GPUS (Graphics Cards) to help Accelerate 3D Applications. (Games, Rendering and all that Goodness)  Basically Using 2 GPUS to get better performance than one GPU.

Crossfire (From the AMD Side is the same thing just a different name [the Process of using 2 GPUs to help Accelerate 3D Applications] (Games, video editing or whatever) same thing using 2 Graphics cards to get better performance.

Basically they're AMD and Nvidia's technology's that allow you to use multiple graphics cards to render the game. it works by 1 graphics card rendering out a frame, and then the other graphics card renders the next frame. This cycle can be used with up to 4 cards. Although its not efficient to use more than 2 cards because games are only made to use 2 cards. You could gain with 3 cards, but most games don't code for that.  

in a few years time they will probably start coding for more than 2 cards huh?

thanks :)

Probably not. Its not a big thing because most people cannot afford 2 cards. Nvidia used to push it but, they and AMD have backed off because no one actually has more than 3 cards. Stuff like rendering machines use it with sometimes more than 4 cards to speed up performance even more. There is a quad Gtx 690 (one  690 has 2 Gtx 680 cores on it) rendering system on YouTube. Here: https://www.youtube.com/watch?v=FPCA34EVoIw

Probably but i doubt people would want to live with 4 GPUS in a System the Power Usage would be insane. and there is a Sacrifice that comes with using an SLI or Crossfire Setup. there are things that can go wrong here are some of them.

  1. The Screen tearing would be insane. and clarify if you also don't know what that is basically think about holding loose-leaf Paper and tearing it in half. basically would you would see is that one paper moves and the other paper stays the same. same thing with computers. when you use more than one GPU it shares the acceleration process from all cards basically one frame comes from one card and another frame comes from another card. if you have TOO many frames lets say you're pulling 130+ frames per second chances are it can tear. basically one frame on the top of the screen screen would stay there and the other frame on the bottom of the screen would keep moving causing a tear. that looks like that piece of paper.
  2.                    Also Power Consumption. think about having 4 Graphics cards in your system you'd probably need more than a 1000+w Power supply you'd run up your light bill with 3 or 4 GPUS. and to give you an example we all know Air Conditioners Run up your light bill during the summer depending where you live. My Air Conditioner during the summer runs up my light bill and the Wattage on my Air Conditioner is 930 watts imagine a computer alone with 4 GPUS pulling more than 1000+ wattage.
  3.                    Also some games are not guaranteed to run. Most modern day games today will. but lets say you tried to run I don't know (Doom or Wolfenstein or Deus Ex)  there's a chance the game won't run with 4 GPU's.

You'd probably be best getting 2 1000 watt power supplys to provide a good amount of headroom.

thats too crazy haha

probably true, would a computer with a 600w psu be a lot of money for your power bill?

no. because your PC what ever you decide to build or buy isn't always going to Use Exactly 600watts, think of the Power Supply like a Power Limit. its enough for a Single GPU overlocked, a Single CPU overclocked and a couple of hard drives. but its not going to use the FULL 600 watts. if you are using a PC and it is Exactly using 600 Watts it would be best to grab a power supply that has more juice like a 750w or a 850w. its good to grab a little more than what you need cause it helps when you are upgrading in the future. you wont have to worry if you upgrade a component that has a wattage that will surpass the power limit on a PSU.

how would I know how much my computer is using? would it be in the BIOS?