Key Attribute : Key is an attribute or collection of attributes that uniquely identifies an entity among entity set.
For example, the roll_number of a student makes him/her identifiable among students.
Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.
Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key.
Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.