Translators

 Translators are low powered FM stations operated for the purpose of re-transmitting an existing AM or FM radio station without significantly changing its content. A complex set of rules govern translator service, including that translators are considered a secondary service and can be forced to change frequencies or leave the air altogether if in the way of a full-service station wishing to upgrade or relocate its transmitting facilities.

Boosters

Boosters are used to re-transmit an FM radio station in order to overcome limitations of terrain or other factors within the FM station’s authorized coverage area.

For more information about Translators and Boosters the FCC has some good information at https://www.fcc.gov/media/radio/fm-translators-and-boosters