A trigger is a stored procedure that instructs the system to take one or more actions when a specific change is attempted. By preventing incorrect, unauthorized, or inconsistent changes to data, triggers help maintain the integrity of a database.
Triggers can also protect referential integrity—enforcing rules about the relationships among data in different tables. Triggers go into effect when a user attempts to modify data with an insert, delete, or update command.
Triggers can nest to a depth of 16 levels, and can call local or remote stored procedures or other triggers.