There are two types of entity sets as given below:
1. Strong Entity Sets
Entity set having any key attributes are known as Strong Entity sets.
Ex. The Figure 2.12 shows strong entity set Student having key attribute Reg_No.
2. Weak Entity Sets
Entity sets having no key attributes are known as Weak Entity sets.
Ex. The Figure 2.13 shows weak entity set Table having no key attributes to identify the tuples uniquely.
A weak entity set is always associated with another entity set to make it meaningful. This another entity set is known as Identifying entity set.
Source: Gupta Satinder Bal, Mittal Aditya (2017), Introduction to Database Management System, 2nd Edition-University Science Press (2017)