check Articles on Trigger:
http://www.mindstick.com/Articles/d9d0f625-6146-415b-b286-3319e3ec336f/?Trigger%20in%20SQL%20server
http://www.mindstick.com/Articles/8a50890e-16b5-4fc6-8970-54ae61a898f9/?Triggers%20in%20SQL%20Server
http://www.mindstick.com/Articles/f7074849-0e9e-463b-a12d-f3d7a35b2785/?Trigger%20in%20SQL%20Server