A township is a geographic land division within a county. (Exampe: Boomer Township is in Pottawattamie County, Iowa.) Prior to 1853, Pottawattamie County was only divided into election precincts and had no distinctive township organization or administration (Frontier Chronicle, Pottawattamie County Genealogical Society). The official definition of a township is discussed on www.wikipedia.org: "The term township generally means the district or area associated with a town. However in some systems, especially in the U. S., no town need be involved. . . . Specific use of the term . . . usually [describes] a local rural or sem-rural government within a county."
A settlement is a specific town within a county, often within a township. (Example: Kanesville is a settlement within Kane Township in Pottawattamie County, Iowa.)