What are Key Constraints,Domain Constraints,Entity Integrity Constraint and Referential Integrity Constraint ?

Domain constraint-the value of each attribute A must be an atomic value from the domain A.
Entity integrity constraint-No primary key can be NULL.
Key constraints –no two tuples can have the same combination of values for all their attributes.Null values are allowed or not.
Referential integrity constraint-it states that a tuple in one relation that refers to another relation must refer to an existing tuple in that relation.Referential integrity constraints are represented by drawing a directed arc from each foreign to the relation it references.