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.