JDO implementations manage the persistence of objects into the datastore. An Extent is a collection of objects of a particular type of object that have been persisted. When you define the MetaData for a class you can define if the class requires an Extent. The default is true. You access the Extent as follows
Extent e = pm.getExtent(MyClass.class, true);
This example will return an Extent that contains all persisted instances of the class MyClass and its subclasses (the second argument in the call). An Extent is useful where you want to restrict a Query to query over just that set of objects. It can also be used where you just want to retrieve all persisted objects of a type (as an alternative to using a Query).