The National Institute of Standards & Technology (NIST) maintains the coding system used by the US Census and other agencies to support consistent referencing of geographic areas in the US. FIPS (Federal Information Processing Standards) codes are defined for states (2 digits), counties within states (3 digits), Census tracts within counties (6 digits), etc. These codes are concatenated to provide unique identifiers for any area down to the US Census Block level. They support efficient database matching, unlike names that may have different versions, e.g. DeSoto vs. De Soto, St. Claire vs. Saint Claire,... All of the official publications relating to FIPS codes are available from the FIPS Home Page
Changes in US County FIPS codes between 1990 and 2000 Census are documented by NIST in FIPS Publication 6-4. For example:
You will be joining external county-level data (typically in Excel) to Arc feature attribute tables, which will require precise matching of values in some field in the external data table to values in some corresponding field in the attribute table. records. If your external data table lacks FIPS codes, you can try creating and matching on combination STATE_COUNTY fields created by concatenating STATE and COUNTY text fields. Of you can match FIPS codes to state and county names, perhaps with Excel's VLOOKUP function.
Table Joins are easily messed up by alternate spellings, capitalizations or abbreviations, leading and/or trailing space characters, etc. Data cleanup is a lot easier in Excel than in Arc. Excel has various text functions with which you can create columns with variant spellings, etc. Excel's VLOOKUP utility lets you match a value across multiplc columns.