hahaha, noooo we're losin another one! BASTEEERD!!
*ahem*
Anyway, your card's an NVidia. NVidia makes the GeForce line of GPU's. GPU is the graphics processing unit, which is the big square chip on the card which says NVidia:
The card itself is usually manufactured by another company, using the GPU from NVidia, and RAM, PCB, etc. from various other companies. The card you have was made by Gainward I believe.
OC means overclock. Overclocking means running your hardware faster than it was intended to be run. The GPU has clock cycles, meaning it can move data a certain number of times per second - this is what the MHz/GHz ratings refer to. For example, my GF4 runs at 300MHz core clock, meaning the GPU cycles 300 million times per second. You can overclock the frequency to make the card run faster -- my GF4 has hit about 333MHz before it craps out, but that's 333 million cycles per second, or a 10% speed increase.
OC'ing takes some time and patience to be done correctly, and you have to take better cooling measures to keep the chip running smooth, but it can pay off in the end.