Strong Entity Sets
The entity types containing a key attribute are called strong entity types or regular entity types.
eg:- The Student entity has a key attribute RollNo which uniquely identifies it, hence is a strong entity.
Weak Entity Sets
An entity set may not have sufficient attributes to form a primary key. Entity types that do not contain any key attributes, and hence can not be identified independently are called weak entity sets.
A weak entity can be identified uniquely only by considering some of its attributes in conjunction with the primary key attribute of another entity, which is called the Identifying Owner entity.