EXCEPT and INTERSECT in T-SQL

The UNION, EXCEPT and INTERSECT operators of SQL enable you to combine more than one SELECT statement to form a single result set. The UNION operator returns all rows.

EXCEPT
Returns any distinct values from the query to the left of the EXCEPT operator that are not also returned from the right query.
INTERSECT
Returns any distinct values that are returned by both the query on the left and right sides of the INTERSECT operator.

The INTERSECT operator returns all rows that are in both result sets. The EXCEPT operator returns the rows that are only in the first result set but not in the second

EXCEPT operator is another most important feature in SQL Server which is used to returns distinct rows by comparing the results of two input queries. Both SQL queries within the EXCEPT query, the number and the order of the columns must be the same in the result sets within similar data types.  EXCEPT operator is a very quick and easy way to find differences, especially when needing to get all differences including null.

When using INTERSECT operator the number and the order of the columns must be the same in all queries as well data type must be compatible.

For more Information check MSDN site