Primary key - Primary key means main key
def:- A primary key is one which uniquely identifies a row
of a table. This key does not allow null values and also
does not allow duplicate values. For ex,
     empno   empname   salary
  1   firoz     35000
  2   basha     34000
  3   chintoo   40000

it will not the values as follows:
1   firoz   35000
1   basha   34000
  chintoo    35000

foreign key - a foreign key is one which will refer to a
primary key of another table

for ex,
  emp_table   dept_table
empno empname salary  deptno   deptno deptname

In the above relation, deptno is there in emp_table which
is a primary key of dept_table. That means, deptno is
refering the dept_table.

Super Key:- superkey is a set of columns within a
table whose values can be used to uniquely identify a row.

A candidate key is a minimal set of columns necessary to identify a row, this is also
called a minimal superkey. For example, given an employee table,
consisting of the columns employeeID, name, job, and departmentID, we
could use the employeeID in combination with any or all other columns
of this table to uniquely identify a row in the table. Examples of
superkeys in this table would be {employeeID, Name}, {employeeID, Name,
job}, and {employeeID, Name, job, departmentID}.

What is a key?

A
key is an attribute (also known as column or field) or a combination of
attribute that is used to identify records. Sometimes we might have to
retrieve data from more than one table, in those cases we require to
join tables with the help of keys.  The purpose of the key is to bind
data together across tables without repeating all of the data in every
table.

Different types of keys?

Usage in Dbms : There are Different kind of keys .

Primary key with example