A contact microphone is one that use vibration of an instrument to pickup its sound. Capsule microphones use the air vibrations, as set up by the sound waves, to pickup the sound, contact microphones use the vibrations set up in the instrument to pickup the sound. A contact microphone will generally be able to produce higher amplified levels before feedback than a capsule microphpone but they can also sound a little harsher than a capsule microphone. They are very good in noisy atmospheres, either due to other background noise or from other very loud instruments. They can be used along side capsule microphones to get the best of both worlds. Accusound manufacture a number of "combined" systems which use one or more capsule microphones along side a contact microphone, these are available for most instruments.