A persistence context handles a set of entities which hold data to be persisted in some persistence store (e.g. a database). In particular, the context is aware of the different states an entity can have (e.g. managed, detached) in relation to both the context and the underlying persistence store.
Although Hibernate-related (a JPA provider), I think these links are useful:
http://docs.jboss.org/hibernate/core/4.0/devguide/en-US/html/ch03.html
http://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/architecture.html
In Java EE, a persistence context is normally accessed via an EntityManager.
http://docs.oracle.com/javaee/6/api/javax/persistence/EntityManager.html
The various states an entity can have and the transitions between these are described below:
http://docs.jboss.org/hibernate/entitymanager/3.6/reference/en/html/objectstate.html
http://gerrydevstory.com/wp-content/uploads/2012/05/jpa-state-transtition.png