GPU of Graphic card

Graphics processing unit
Graphics processing unit or GPU

1. Atkin D., 2011, The Right GPU for You.
Your PC's graphics processing unit (GPU) is its second-hardest-working component, after the CPU. If you're like most PC buyers, however, you probably haven't given it much thought. Sure, if you play any games on your PC more sophisticated than FreeCell or Bejeweled, choosing a graphics card is probably the first thing you do when buying or upgrading a PC, because graphics performance makes all the difference for gaming. But if you don't play 3D games, edit video, or use computer-aided-design (CAD) software, you've had little reason to think about graphics performance.
2. Wikipedia, 2011, Graphics processing unit.
A graphics processing unit or GPU (also occasionally called visual processing unit or VPU) is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display. GPUs are used in embedded systems, mobile phones, personal computers, workstations, and game consoles.
The GPU is a CPU of graphic card because has a brain of the card.
3. Volodymyr V. Kindratenko , Jeremy J. Enos , Guochun Shi , Michael T. Showerman ,Galen W. Arnold , John E. Stone , James C. Phillips , Wen-mei Hwu, 2009, GPU Clusters for High-Performance Computing

Commodity graphics processing units (GPUs) have rapidly evolved to become high performance accelerators for data parallel computing. Modern GPUs contain hundreds of processing units, capable of achieving up to 1 TFLOPS for single-precision (SP) arithmetic, and over 80 GFLOPS for double-precision (DP) calculations. Recent high-performance computing (HPC)-optimized GPUs contain up to 4GB of onboard memory, and are capable of sustaining memory bandwidths exceeding 100GB/sec.

4. Wikipedia, 2011, Graphics processing unit(GPU forms).
The GPUs of the most powerful class typically interface with the motherboard by means of an expansion slot such as PCI Express (PCIe) or Accelerated Graphics Port (AGP) and can usually be replaced or upgraded with relative ease, assuming the motherboard is capable of supporting the upgrade. A few graphics cards still use Peripheral Component Interconnect (PCI) slots, but their bandwidth is so limited that they are generally used only when a PCIe or AGP slot is not available.
A dedicated GPU is not necessarily removable, nor does it necessarily interface with the motherboard in a standard fashion. The term "dedicated" refers to the fact that dedicated graphics cards have RAM that is dedicated to the card's use, not to the fact that most dedicated GPUs are removable. Dedicated GPUs for portable computers are most commonly interfaced through a non-standard and often proprietary slot due to size and weight constraints. Such ports may still be considered PCIe or AGP in terms of their logical host interface, even if they are not physically interchangeable with their counterparts.