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.
Here are the major Census geographical terms and their definitions.
Here are some more terms dealing with urban areas:
While not as challenging as comparability problems arising from differences in questions asked, geographical changes can be very important nonetheless.
Metropolitan areas. As you can imagine, in the ten years between Censuses, new cities will attain Metropolitan Area status (although the changing definitions applied to urban areas account for some of the differences, too. And the boundaries of MAs will shift considerably over time as well, making comparability of data for some applications difficult.
Census tracts. Tracts are also variable over time, since shifts of population concentrations may necessitate new boundaries.
Even such supposedly stable geographic entities as counties can be changed over time.