Shape keys are modifications of the original mesh that you can animate and mix with each other. Previously called Relative Vertex Keys (RVK), one of their major uses is to create facial expressions.
Shape keys store different shapes of the same mesh. In other 3d applications they are called 'morph targets' or 'blend shapes' or even 'vertex keys' in older versions of Blender.
Shape Keys are the specified positions of vertices within an Object; the actual points that define the mesh. Since this can involve thousands of vertices for an object, separate motion curves are not created for each vertex because it would overload your computer's memory.
Let's get into some more details here...
The Blender API provides us with the means to define IPOs from scratch, enabling the definition of movements not easily re-created by setting key frames by hand. Thus there is more to IPOs than just driving one IPO by another one. Furthermore, Shape keys and poses are examples of (collections of) IPOs that are quite different from, for example, a location IPO. We will encounter both shape keys and poses later on in this article, but we will start off with looking at how we might define an IPO from scratch.
This Blender tutorial is the fourth in my game engine tutorial series. This one will teach you how make a collectible key that you will then use to open a door.