PlayDiplomacy Player Classes are groups of players in particular classes with particular capabilities.
The best known is probably the Premium and non-Premium classes; Premiums can play variants, more than three games at once etc.
Two (or perhaps two and a half) new classes are Diplomats / Ambassadors and the honorary Star Ambassador title. These are discussed in their own thread, but basically Ambassadors can set up games that are only open to Ambassadors if they choose.
Future classes have yet to be implemented, but may include making Star Ambassadors a class of their own, adding a Schools class, etc