SLi can be done with Nvidia PCi-Express GRAPHICS cards.
To put it simply, one card renders the top half while the other renders the bottom half or the display.
You have twice the graphics power(theoretically).
So all in all, you're going to have a more powerful card than the you currently use (depending on what card you use now.. lol) plus another one on top of that.
It makes it easier by spreading the work load across 2 GPU's, similar to DUAL CPU etc.
Now whether the SLi feature will function with all software on your machine or just with some i'm not sure. That will probably be dependant on the driver settings???
Hope this helps.