Crossfire and SLI are two types of multi-GPU or multi- Graphics Processing Units which are mostly used to connect multiple video game cards in order to ensure high performance or faster animation when playing video games. Both crossfire and SLI use two or more graphics, and this makes it easy for gamers to connect two or more low-end cards that are very cheap instead of wasting resources on high- end cards that are very expensive.
However, if you can afford two high-end cards, you can connect them together using crossfire or SLI. Both multi-GPU technologies are produced by different companies, and that is one of the differences between the two. SLI was created by Nvidia, and it can only connect two or more cards that are produced by Nvidia. On the other hand, crossfire was created by ATI, and it can only connect two or more cards that are produced by ATI.