County divisions, as the name implies, are used by the United States Census to collect statistics at a level below that of a county for the 50 US States, the District of Columbia, and several outlying territories.

The type of entity varies from state to state:

The source for most of this writeup was the Glossary of geographic terms for the US Census. It listed these terms alphabetically and didn't always say where they were used. That took a bit of research.
1Yes, this is an official term of the US Census.