What is an Entity?
It is a 'thing' in the real world with an independent existence.

What is an Entity type?
It is a collection (set) of entities that have same attributes.

What is an Entity set?
It is a collection of all entities of particular entity type in the database.

What is an Extension of entity type?
The collections of entities of a particular entity type are grouped together into an entity set.

What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.