You can see the list of all classes here.
An Entity is the base class for characters, rooms, objects and areas. None of the C++ fields are really accessible from ARC. In the c++ code, the class holds things common for all those subclasses: events, the ability to print itself out etc.
A Character is a mob or a player.
A Player class holds extra data that only players have. You get to it by using the pcdata member of the Charcter class.
An Account is the data common among this player's characters. It's accessible via the account member of the Player class.
Other self-explanatory classes:
You can also access psuedoclasses like Class and