The benefit is that you have two GPU's (Mind you both cards have to be the same model). Each GPU renders one half of the screen, thus each of them share the work which puts less strain on each of them. Since there is less strain they can use that extra power to make your games run faster, its not quite twice as fast as if you only had the one card in most conditions though.
The bad is that since you have two cards they are going to be more power hungry and usually you can buy a better single video card for the same price as two SLi cards.
Crossfire is ATI's version of dual GPU's
To use SLi or Crossfire you need a motherboard which supports it and has 2x PCI-E slots and 2x SLi or Crossfire ready cards and the bridge or wire that connects them together.
