FIPS Codes are 5-digit codes that are assigned to each county.   The first two digits are for the State and the last three identify the County, or county equivalent.  So, each State has its own 2-digit number, and each County within the state has its own 3-digit number, which are combined into a 5-digit number to uniquely identify every US county.  For Example: Connecticut's state number is 09, and New Haven county, CT is 009, so New Haven county's FIPS code is 09009.  FIPS Codes are easier to handle in information systems than are county and state names.

FIPS Code County
009001 Fairfield
009003 Hartford
009005 Litchfield
009007 Middlesex
009009 New Haven
009011 New London
009013 Tolland
009015 Windham