Windows 7 includes a new type of service called trigger. A trigger-start service is a regular service that you can configure to run (or stop running) only when certain criteria and conditions that you define are met. Some conditions would be:
Device arrival or departure
Joining or leaving a domain
Opening or closing a firewall port
Group policy change
To view trigger information, use the sc command with the parameter of qtriggerinfo.
Certain services will not have any triggers present.