Persistence classes are used to manage "persistent data". Persistent data is the only kind of data in the system that is permanent. In other words data that is stored in the database.
The purpose of persistent objects is to abstract the physical data representation (i.e. database tables, rows, columns).
Persistence is the storing of a message to a database (in AFW) so that it is restartable. Asynch messages are persisted, synch are not.