Questions related to using and manipulating Shape Keys. Shape keys are used to deform objects by transforming the locations of control points over time. There are two kinds of shape keys, Relative and Absolute.
Shape keys are used to deform objects into new shapes for animation. In other terminology, shape keys may be called “morph targets” or “blend shapes”.
The most popular use cases for shape keys are in character facial animation and in tweaking and refining a skeletal rig. They are particularly useful for modeling organic soft parts and muscles where there is a need for more control over the resulting shape than what can be achieved with combination of rotation and scale.
Shape keys can be applied on object types with vertices like mesh, curve, surface and lattice.
See more information at the Blender Manual