An Entity
EJB is a temporary object-oriented representation of a business record (aka a record in a relational database).
The record in the database is a representation of a business entity, e.g. account, customer, product, etc.
A Session EJB represents a business process that involves zero or more (business entities | records | Entity EJB).
To understand the transactional nature of each type of EJB and the EJB container, you need to understand the concept of a "transaction" in terms of business computing software.