Nvidia is the main manufacturer of any nvidia card,like AMD is a manufacturer of its own AMD card.When we see any card e.g.-Written 'Nvidia geforce GTX 980' on box of card specially written 'Nvidia',We call that a reference card of Nvidia or which is made by the manufacturer itself.
Zotac,Asus,MSI,Gigabyte,EVGA,Galaxy,XFX are some companies they use 'Nvidia' technology and produce their own many variant of a single graphics card.Like Zotac GTX 980 AMP Extreme edition,Zotac GTX 980 AMP OMEGA edition are 2 variants of GTX 980 over main Nvidia GTX 980.But the producer is Zotac.On the box of these card we find the name Zotac Geforce GTX 980 insted of Nvidia Geforce GTX 980.Those cards made by these companies Zotac,Asus.......etc are called...