It's AMD's flagship GPU. AMD, like Nvidia, doesn't make the cards themselves (apart from engineering samples). They ship their chips to the other manufacturers such as Asus, Gigabyte, Msi, etc. The only real difference between all of these is usually the cooling solution.
Advantages of having 2 or more graphics cards is obviously better 3D performance which is aimed at gamers with deeper pockets. The disadvantages are the extra cost and power usage, seldom driver issues, game incompatibility and microstutter. It's usually reserved for high end parts since it's pretty much always better to have 1 flagship card (Radeon 7970, GTX680) than 2 mid ranged cards (Radeon 7850, GTX660) to avoid pretty much all of the above mentioned disadvantages 100% of the time.
To check if your system supports CrossFire (AMD)/SLI (Nvidia), it's usually best to refer your motherboard's manual or specification sheet on manufacturer's website. Pretty much all Intel based motherboards with 2 or more PCI-e x16 slots support both CF and SLI but some AMD based boards only support CF due to licencing issues.