A process class is defined as a class which encapsulates a business process. It is a collection of operations. Process classes do not have data maintenance operations, or indeed any relationship with database tables. Instead they manipulate other entity and process classes in order to implement a business process.
For example, in a banking system, you could have an account transfer process which debits money from one account and credits another. In this case, internally, the process would use the above Account entity class to update one account to debit it, followed by an update of the other account to credit it. Note that the process class itself does not do any database manipulation, it merely packages a sequence of entity operations in order to carry out the business process modelled.
Process classes have a stereotype of process.