It is important to understand the geography of the Census because different items of information are available for different units of geography. Simply knowing the "normal" geography of an area isn't always sufficient since the Census Bureau defines some geographical areas using different boundaries and different terminology. The Census Bureau does attempt to make its geographical areas conform to what an area is really like, however, and they use local input whenever possible.
Census Tracts are the smallest areas for which the important income, education and other social data (derived from surveys) items are available. Blocks are the smallest area for which the Census reports data, and here the items available are largely the basic ones from the 100% counts: age, sex, and race.
An important reason for this is confidentiality. A fundamental Census principle is that they will not display any data that can be used to identify a particular person, household or other basic counting unit. There might very well be so few people in an income class, for example, in a certain block, that reporting the numbers would actually give information on a particular household.
NOTE: much of the information on this page comes from an informative talk given by Josh Coutts, Census Bureau: "U.S. Census Bureau Geographic Entities and Concepts."
Here are the major Census geographical terms and their definitions. For more definitions, see Geographic Terms and Concepts at Census.gov.
Here are some more terms dealing with urban areas:
There is not always a one-to-one, or even a nested, relationship among all Census geographical entities. For example, a Census Tract may be split between two different cities. (However, Tracts are never split between different counties.) When this occurs, it's just not possible to simply get data for all the Census Tracts within a city.
Keep in mind that sometimes customary names for places and areas (for example, names of post offices) might not correspond with a legal place boundary. And even legal place boundaries might not correspond exactly with Census geography. Use some of the reference source in boxes on the right to be sure you are looking in the right place for the data you need.
TIGERweb from Census.gov is a great tool for exploring geographical data and making sure you are looking at the right geography to answer your question.
Here is a summary of the distinction between legal geographic entities and those that are wholly creations of the Census Bureau for statistical purposes.
Legal geographic entities (Census Bureau does not determine the boundaries, but uses them):
Statistical geographic entities (Census Bureau determines the boundaries, although in cooperation with governmental agencies):
NOTE: For help with Texas geographies, here are some resources from the Texas Demographic Center (formerly the State Data Center).
TIGER. This catchy acronym stand for Topologically Integrated Geographic Encoding and Referencing system and is the technical basis of all Census geography.
Details about TIGER or other Geographic Information Systems (GIS) are beyond the scope of this LibGuide. But more information is available from Census.gov (below) or in other resources listed under the "Other Guides to Census Information" tab.