it is enough to say that entities are (generally application-specific) classes which correlate to rows in a table. Specifically they correlate to the row by means of a unique identifier. Because of this unique identifier, entities exist independently and define their own lifecycle. As an example, when we delete a Membership, both the User and Group entities remain.
We need to remember:This notion of entity independence can be modified by the application developer using the concept of cascades. Cascades allow certain operations to continue (or "cascade") across an association from one entity to another.