There are mainly two types of keys are available in SQL server
1. PK
2. FK

1.PK-- In a Table the unique key(with no repetation and does
nota take null value) is called Primary Key 
2.FK-- Foreign Key when that PK refers to another table then
its called as foreign Key

keys-A key allows us to identify a set of attributes and 
thus distinguishes entities  from each other.keys also help 
uniquely  identify relationships,and thus distinguishes 
relation with each other.
different types of keys:
(i)super key
(ii)candidate key
(iii)primary key
(iv)unique key
(v)foreign key
sk-it is a set of one or more attributes that allows us to 
identify uniquely an entity set.
ck-a super key for which no subset is a superkey is called 
a candidate key.
pk-In a table pk does not take no repetation and does not 
allow null values.
fk-it acts as a referential order.it refers one table from 
the another one.