exact difference between cpu gaming, single graphics card gaming and crossfire.
With 'cpu' gaming, I would assume that is using the integrated graphics which is usually very poor performing and only for basic 3d applications/gaming.
With a single card, this can vary greatly from card to card however it means that you are using the graphics card to do the work instead of integrated graphics.
Crossfire/SLI is when there are 2 or more graphics cards accelerating the application at the same time.
Most people simply use 1 card because it is sufficient for almost all tasks. Integrated may even be enough depending on what you use it for. Multi-GPU is usually for enthusiasts, extreme gamers.
Cpu gaming is based on the direct usage of processor in which the game is target with only RAM & processor or if integrated graphics card is present then it also uses it power.
Single graphics card gaming is using of single discrete gpu for gaming intensive application which is much more reliable then cpu gaming as well as much more faster in performance and quality.
Crossfire is a term used for the gpu of AMD cards.In this you can pair two different gpu's or similar pair of gpu's so that the gaming experience is on par in performance & quality.It is generally used up when a user really want to play at multiple monitors or at a very high resolution like 2560x1600 and above.
SLI is used in Nvidia.It will require similar pair of graphics card to be in SLI mode.
The only disadvantage of SLI mode is it requires two pair to be of equal specification capacity inspite of any company build to be in SLI.