Meshes

The geometry of the models are called a mesh, and are made of polygons, or 2D shapes: triangles (3 sides), quads (4) and n-gons (5+, typically undesirable).

These polygons are made of vertices (the points), edges, and faces.

Originally posted by ck12.org:

Topology

The makeup of those polygons on a mesh is called the topology. Good topology is essential to working on models effectively.

Normals

Every polygon has a normal vector, perpendicular to its coplanar 2D plane. This helps determine what parts of the mesh are "inside" and "outside".

By default Unity does not render the back faces of polygons, called backface culling.

If you can only see the back faces of your avatar (or a part of it), chances are the normals are inverted. This will be especially noticible; it happens to a lot of avatars when custom shaders are disabled.