Trigger-Started Services are new with Windows Server 2008 R2 and Windows 7. Before, services were typically started at system boot time and ran until system shutdown. Now, services can specify start and stop conditions (triggers).
Common usage scenarios include:
Trigger meta-data is stored in the service registry key. You can use the “sc qtriggerinfo” command to view service triggers.
Watch the new MSDN Channel9 episode to learn more.