Generalized area representations of U.S. Postal Service (USPS) ZIP Code service areas. Simply put, each one is built by aggregating the Census 2000 blocks, whose addresses use a given ZIP Code, into a ZCTA which gets that ZIP Code assigned as its ZCTA code. They represent the majority USPS five-digit ZIP Code found in a given area. For those areas where it is difficult to determine the prevailing five-digit ZIP Code, the higher-level three-digit ZIP Code is used for the ZCTA code. ZCTAs do not precisely depict the area within which mail deliveries associated with that ZIP Code occur. A five-digit ZCTA ending in "HH" (e.g., "006HH") represents the water area within a three-digit ZCTA that is not associated with any mail delivery route. A five-digit ZCTA ending in "XX" (e.g., "006XX") represents the land area within a three-digit ZCTA that is not associated with any mail delivery route.
Source and for further information: U.S. Census Bureau http://www.census.gov/geo/ZCTA/zcta.html