FIPS county codes are a Federal Information Processing Standard for three-digit codes used to uniquely identify a county within a US state (?? not sure about how/whether the idea of county fips applys to other nations. ??) This can be paired with the two-digit us FIPS state code to form a five-digit FIPS code that uniquely identifys a county within the entire United States.

County FIPS codes in the United States are usually (with a few exceptions) in the same sequence as alphabetized county names within the state. They are usually (but not always) odd numbers, so that new or changed county names can be fit in their alphebetical sequence slot.

Lists of state and county FIPS codes for the United States can be found at http://www.epa.gov/enviro/html/codes/state.html