NVIDIA is a technology company that makes graphics cards, which are normally a constant feature of computer games; they are responsible for increasing the quality of the gaming visual appearance on a computer screen.
Nvidia GT cards are basically made for everyday casual use, and some light-medium gaming such as Half-Life 2, Far Cry 1 & 2, Need for Speed: Underground 2 and so on, and perhaps basic video editing such as in Sony Vegas Pro 10 when making a “Let’s play” video. They are not really meant for high-end gaming, as it will have a hard time handling GTA V.
Nvidia GTX cards, on the other hand, can handle more complex stuff because they're meant for more professional use, geared towards medium to high-end gaming, especially the GTX 1080, and very complicated works such as video editing in 4K resolution, with a lot of compositing, effects, and similar things. Not to mention rendering full 3D animated movies. Nvidia GTX cards will effortlessly handle GTA V.