Shape Keys are used to animate deform objects into new shapes. They can be applied on objects with vertices like Mesh, Curve, Surface, Lattice.
There are two types of Shape Keys:
Which are relative to the Basis or selected shape key. They are mainly used as, for limb joints, muscles, or Facial Animation.
Which are relative to the previous and next shape key. They are mainly used to deform the objects into different shapes over time.
The shape key data, the deformation of the object’s vertices, is usually modified in the 3D View by selecting a shape key, then moving the object vertices to a new position.