basically, its a little electric valve that routes air back and forth to move the bolt system.....www.zdspb.com
has a good write up on what lots of parts are and how they work.
its kinda hard to blow one, what gun do you have? what happened? and what did you do to make it happen? and where is it leaking from?
they're kind of expensive so its a good idea to make sure its not the noid, however most markers have some sort of seal near the noid that can make people think the noid is blown when its just an o-ring that needs to be changed instead.