A GPU, or Graphics Processing Unit, is a special type of processor used in computer graphics hardware (it is part of the graphics card).
The term GPU is often used interchangeably with Graphics Card or Video Card.
The primary purpose of a GPU is to render 2D and 3D graphics
VRAM (video RAM) is a reference to any type of random access memory (RAM) used to store image data.
Your graphics card comes with RAM which is referred to as VRAM.
The more VRAM you have, the larger capacity you have to store these complex computations.
Here's the Definition of VRAM by Techtarget.com