Ballotpedia is an online Encyclopedia of American Politics and has state and county pages for the 50 states in the US. There are explanations of the different government and political divisions in each state and county (parish/borough) as well as explanations of the political and government administration at the local level from cities and towns to villages/gores, charter vs. general law, court systems, and more.

This is a useful site for a quick lesson or refresher on the divisions and types of government in areas of the United States you’ll research.