The U.S. Citizenship and Immigration Services (USCIS) is a federal agency operating under the dept. of homeland security. The federal agency provides its services through its official website, offices spread across the country and overseas, and its phone service.
The USCIS site provides many online services and detailed information on several topics including, green card, citizenship, form filing fees, immigration options, case processing times, case updates, etc.
Field offices handle the interview process for all non-asylum seekers. Besides interviews, the offices also provide naturalization ceremonies and services that support the online website.