U.S. Census Statistics: Census Geography

Describes in great detail the basic geographic entities the Census Bureau uses in its various data tabulations and documents the purposes, definitions, standards, criteria, and procedures used to select, define, delineate, and revise these geographic entities up to 1990.

Before 1980 not all counties were divided into tracts. In 1990 and 2000 all counties were fully tracted. This lists the tracts as they were created.
Source: NHGIS

Information on current and recent geographic programs, areas and definitions.

Diagrams, charts, manuals and map files.

Metropolitan and micropolitan statistical areas are the most frequently used for studying cities and urban areas.
Current and historical definitions, maps and lists.