The Public API module rounds up the most commonly used classes of PUN.
The classes which are most commonly used, are grouped into a Public API module, which is only a documentation structure. Classes like Photon.Pun.PhotonNetwork and Photon.Pun.MonoBehaviourPunCallbacks are good entry points to learn how to code with PUN.
Typically, classes for internal use are not public but there are a few exceptions to this where access may be of use, if you know what you're doing.