The PersistentStore and PersistentObject classes work hand-in-hand to get access to data in the PersistentStore. All of the methods in the PersistentStore class are static, which means that you won't ever create an instance of the PersistentStore class. Instead, the PersistentStore class primarily exists so that you can get a PersistentObject, which in turn allows you to get your data.
A PersistentObject represents the object being stored in the PersistentStore. It is a wrapper class that handles the serialization and storage of the data for you. As a wrapper it is possible for a PersistentObject to actually reference no object or to have a null value. This is the case when a new PersistentObject is created.
You can store as many instances of PersistentObject in the PersistentStore as you want, limited by device memory of course. Each object in the PersistentStore must be given a unique key value by you and this key value is used to reference the object...