This is the API documentation for Eigen.
For a first contact with Eigen, the best place is to have a look at the tutorial.
Most of the API is available as methods in MatrixBase, so this is a good starting point for browsing. Also have a look at Matrix, as a few methods and the matrix constructors are there. Other notable classes for the Eigen API are Cwise, which contains the methods for doing certain coefficient-wise operations, and Part.
In fact, except for advanced use, the only class that you'll have to explicitly name in your program, i.e. of which you'll explicitly contruct objects, is Matrix. For instance, vectors are handled as a special case of Matrix with one column. Typedefs are provided, e.g. Vector2f is a typedef for Matrix<float, 2, 1>.
Most of the other classes are just return types for MatrixBase methods.