The first link on the site provides the File Layout. It is based on 1997 geography and contains estimates of counties by state in 5-year age groups. The age range starts with the 0-4 range and goes through 85 years and older. The Hispanic Origin character group is modified every July 1, dating back to 1990. For an explanation of the data and the data links, the link to Documentation gives the full- body text background.
Every source is noted here.
The third link is FIPS or Federal Information Processing System Codes for States and Counties. The release date for this information is December 1996 and it was last updated April 2000. These numbers uniquely identify geographic regions with two digits for states and five digits for counties. Well over 3,000 counties are identified in the list. The fourth link on the site is for an Explanation of Race and Hispanic Origin Categories.