Often people may become victim to a crime but have no idea who committed it or even if they do know, can often not prove it. CCTV is a great way to obtain the evidence you need to catch someone in the act of an illegal act.
For you to be able to use your CCTV in a court case you need to have abided by certain laws, which many of us simply do not (often because we were not aware we needed to). All images must be taken in compliance with the data protection act.
There are hundreds of CCTV systems on offer, from the basic fixed camera that records on to a tape to Wireless IP cameras allowing you to access and view footage from anywhere with an internet connection. They range in price drastically but often you do get what you pay for when it comes to security.