Sanborn Maps are fire insurance maps. The company has been mapping since 1866 and has over 1.2 million maps.
The maps indicate the buildings in towns and are color coded with information about the structure of the buildings, the main purpose of the maps (fire insurance). They also indicate the street names as they were as of the date on the map. Street names change. Streets disappear. The maps may provide information on where the street was located when your ancestor lived there or show you where a street you’ve never heard of was on the date of, say, a census. Each town may have multiple maps and they may not be the same in each collection.
Detailed information on Sanborn Maps at the Library of Congress and the maps in general may be found here.
The Library of Congress has a collection of Sanborn Maps but it is NOT complete. ProQuest, which is available at many public libraries, also has a collection and it may include different maps. There are other sources, many state specific. Illinois has one at the Consortium of Academic and Research Libraries in Illinois (CARLI). Since the years for the map collections vary you will want to check both. Links are below. You must have a login from a member institution to open the CARLI maps.