Topology!

Topology defines the layout of a mesh surface. It is important to get all of the topology correct in order to animate any object properly without deforming and producing breaks within the mesh.

Topology around areas which are most frequently animated need to be accurate and extra detail must be focused on. The most important areas on the face to animate are the, eyes, mouth, nose, and laugh lines. The way this is achieved is by using edge loops, which help determine where muscle structure or natural contours occur.

A good example of where to use edge loops. (Credit to crazyfool on polycount.com)

 Quads are best to keep to within a mesh in order for the animator to easily work with the model. While tris are not too damaging to the mesh they should be avoided when possible as they can cause problems within certain programs; although they can sometimes be useful to link areas together. That being said tris should not be used in areas which are going to be animated or highly visible. 5poles should also not be used within animated areas as they can cause trouble; however they are useful when changing the direction of the mesh, where two edge loops meet for example.

A simple mesh which includes quads, edge loops, 5poles and tris. (Credit to crazyfool on polycount.com)