The ESP Guitar Company

a trigger is a small device that fits on the rim edge of a drum and a pad rests on the head of choice, when that membrane it struck it fires the trigger sending a signal to a synth sequencer (a brain, or "module" whatever you want to call them)

a trigger itself does not produce a sound, you have to have a module to create the sound

our drummer uses triggers on his bass drum live. but has them set up to where he still has to hammer his kick drum hard enough to trigger them. they sound awsome through a good PA and can be worth the money if venues are using shitty mic set ups or have a shit sound guy. he is using an alesis module with a rolland trigger i think. cos him around £150 on the bay.