GPU stands for graphics processing unit. Although it works in conjunction with a computer's motherboard and central processing unit, it houses its own processor, input/output system and random access memory on a separate circuit board.
The GPU requires this hardware because the calculations involved in graphics rendering are varied and complicated. The GPU governs all the geometric shapes and the actions they perform on a monitor, and some high-end units are larger and more powerful than the computer's CPU. The GPU's RAM is used entirely for storing the many bytes of data in images and video before they are displayed onscreen.