What are database triggers? How are the triggers fired?

Read this collection of questions and answers on SQL Server triggers:

A collection of 20 FAQs on creating and managing triggers. Clear explanations and tutorial exercises are provided on creating DML and DDL triggers; altering, dropping, enabling and disabling triggers; accessing inserted and deleted records; overriding DML statements. Topics included in this collections:

1. What Are Triggers?
2. What Are the Basic Features of a Trigger?
3. How To Create a Simple Table to Test Triggers?
4. How To Create a DML Trigger using CREATE TRIGGER Statements?
5. How To Test a DML Trigger?
6. How To List All Triggers in the Database with sys.triggers?
7. How To Modify Existing Triggers using "ALTER TRIGGER"?
8. How To Delete Existing Triggers using "DROP TRIGGER"?
9. How To Get the Definition of a Trigger Back?
10. How To Disable Triggers using "DISABLE TRIGGER"?
11. How To Create a Trigger for INSERT Only?
12. How To See the Event List of an Existing Trigger using sys.trigger_events?
13. How To Access the Inserted Record of an Event?
14. How To Access the Deleted Record of an Event?
15. How To Improve the Trigger to Handle NULL Values?
16. What Happens to a Trigger with Multiple Affected Rows?
17. How To Override DML Statements with Triggers?
18. How To Create a DDL Trigger using "CREATE TRIGGER" Statements?
19. Can You Roll Back the DDL Statement in a Trigger?
20. Can You Create a Logon Trigger in SQL Server 2005 Express Edition?