What is a Face Recognition System?
How does it work? I want an answer in detail

Generally, they use bone structure and feature positioning.

Here's what seems a couple good resources to learn more:
http://www.face-rec.org/

http://www.frvt.org/EvalMethod.htm

What is a Face Recognition System?
How does it work? I want an answer in detail

Just want to point out that a medium like this is not usually suited to answers in detail. Especially for such a broad question. Most of the people here volunteer their time to help. We usually don't have the time to go into a lot of detail. If you want detailed answers, there is a wealth of research material on the WEB. Google or any of the other search engines can hell you find detailed references.