ExecuteScalar and ExecuteNonQuery

ExecuteScalar - Returns only one value after execution of the query. It returns the first field in the first row. This is very light-weight and is perfect when all your query asks for is one item. This would be excellent for receiving a count of records (Select Count(*)) in an sql statement, or for any query where only one specific field in one column is required.

ExecuteNonQuery - This method returns no data at all. It is used majorly with Inserts and Updates of tables. It is used for execution of DML commands.