Caller ID is a feature that allows you to know who is calling you before answering the phone. Normally, the number of the caller is displayed on the phone. If you have a contact entry for the caller in your contact list, their name appears. But that's the name you have entered in your phone. You can see the name of the person as registered with his service provider, by subscribing to a flavor of the caller ID service called caller ID with name.
Caller ID is also known as Calling Line Identification (CLI) when it is provided through an ISDN phone connection. In some countries, it is called Caller Line Identification Presentation (CLIP), Call Capture or Caller Line Identity (CLID). In Canada, they call it simplyCall Display.
Caller ID is useful whenever you want to 'declare absent' in situations where you receive calls from people you don't want to answer. Many people find this useful when their boss calls. Others might choose to ignore calls from their ex-boyfriend/girlfriend or any hassling person.