Click on the title today Monday Maps, then bookmark on your toolbar the mapping page that comes up -- it's an essential resource.
Because most records in the United States are kept at the county level, it is essential to know what county your ancestor lived in during a particular time. Their records will be found at the county seat of the county they lived in AT THE TIME the records were made despite earlier or later county boundary changes.
Boundary lines for towns, counties, and states evolve --e.g., some counties simply disappear or are are gobbled up and renamed as land in nearby towns or states -- we can lose our family records despite our ancestor never having moved.
My immigrant Cresap ancestor, Thomas, lived in Oldtown, Maryland for over 40 years. During that time he lived in the Maryland western territory, Frederick County, Washington County, and finally Allegany County. All that time he was in the same house on the same meadow.
First find your town either on Wikipedia or using the "Place" search on the The Family History Library catalogue page http://www.familysearch.org/eng/Library/FHLC/frameset_fhlc.asp I have the library catalogue bookmarked on my toolbar next to a folder marked maps where I keep the 101 site and other marvelous maps. The place search is a quick way to find what county a town is in.
Then use the FREE AND ESSENTIAL online map resource from Family History 101 at http://www.familyhistory101.com/map_county.html This is a great website for many reasons, but just play with the maps. Go to your state, pick a year, watch the county boundary changes, and find your folks.