A superkey is defined in the relational model as a set of attributes of a relation for which it holds that in all instances of the relation there are no two distinct tuples that have the same values for the attributes in this set. Equivalently a superkey can also be defined as those sets of attributes of a relation upon which all attributes of the relation are functionally dependent.
A superkey is a column (or set of columns) that can be used to identify a row in a table. A key is a minimal superkey. For example, the employee table. We could use the employeeID and the name together to identify any row in the table. We could also use the set of all the columns (employeeID, name, job, departmentID). These are both superkeys.
However, we don't need all those columns to identify a row. We need only (for example) the employeeID. This is a minimal superkey—that is, a minimized set of columns that can be used to identify a single row. So, employeeID is a key.