What's a trigger?
A drum trigger is an electronic device that converts vibration into a electric pulse. The drum trigger is attached to or build in the drum.
The trigger is connected to a drum-module. This drum-module produces the drum sounds. In most cases a piezo element is used as a transducer.
Acoustic drum kit
From this drum-module, you are able to assign various audio samples to each drum. This way you can turn an acoustic drum set into an electronic drum set. Your electronic kit now has normal tom sizes and looks much better then a electronic kit.