Trigger started services are services in Windows 7 or Windows Server 2008 R2 that can register to be started or stopped when a trigger event occurs. This eliminates the need for services to start when the system starts, or for services to poll or actively wait for an event; a service can start when it is needed, instead of starting automatically whether or not there's work to do.
Examples of predefined trigger events include arrival of a device of a specified device interface class or availability of a particular firewall port. A service can also register for a custom trigger event generated by an Event Tracing for Windows (ETW) provider.
In this screencast, Kate Gregory provides an overview of trigger started services on Windows 7 via the Windows API Code Pack, a source code library that can be used to access some features of Windows 7 and Windows Vista from managed code.