When somebody is calling, usually your mobile phone will display the caller’s phone number and name. If you assigned a photograph to the caller, you’ll see that too. Generally no further details are available.
If you have an Android phone, here is a new app called PhoneTell that can display much more info about the caller like their phone number, location, etc.
The application makes use of your contacts list information and publicly available information for this purpose. That means, most probably, you’ll get more info about the caller even if the caller isn’t in your contact file.
Moreover, the application is not just for displaying callers’ information; with a click you can send the caller a short SMS if you decline their call which lets them know you’re busy. PhoneTell can even remind you to return the call.