Do you need it for SCBCD ? In this case I would suggest reading the corresponding part in PRO EJB 3 or in the ejb 3.0 spec (persistence pdf), because the topic is really too complicated to be described in detail here. And a brief explanation like "a persistence context is some sort of a cache for entities" would probably not suffice for SCBCD.
A persistence context is a set of managed entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed by the entity manager.
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