A database trigger is typically a business rule or data dependency that must be maintained. The trigger is essentially a stored procedure that is activated each time some condition is met (typically a row being inserted into the database, a field being modified, a row deleted, etc.). The trigger can do pretty much anything you want it to do -- insert row(s) into another table, update tables, etc. The main advantage of triggers is that they exist very close to the database, so that no matter how the database is accessed (interactive SQL, applications, etc.), data integrity and consistency can be maintained -- i.e., triggers remove the responsibility from the application and place it in the database itself, where it belongs.
Hope this helps.
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