From BlenderWiki

BMesh

These are my own notes about the BMesh structure and the functions that operate on it. See also The official doc page. and Dev:2.6/Source/Modeling/BMesh.

Concepts

The base concepts are vertex, edge, and face (sometimes called polygon) - these are the things that are selected in the corresponding edit selection modes. There is another concept called a loop. You can think of a loop as a "face corner" - that is, it holds a vertex of the face and pointers to the next loop and also the corresponding edge.

Here's a picture showing how vertex, edge, face, and loop point to each other.