Race
The Census asks individuals to identify their race and ethnicity. In 1990, respondents could choose from only five racial categories; in 2000, from 63 since Census 2000 allows for individuals to identify as more than one race. The list of Race and Ethnic Groups (alone or in combination) includes hundreds of possibilities. Racial and Ethnic Classifications Used in Census 2000 and Beyond explains why the Census made these changes and how they will be tabulated into the six basic OMB categories of American Indian or Alaskan Native, Asian or Pacific Islander, Black, White, Hispanic (of any race), or Some Other Race. Project Race provides analysis from the multiracial community about this. This change has been extremely controversial and has also meant that the data from 2000 and earlier Censuses are not comparable, OMB issued guidelines on how responses of more than one race should be allocated for monitoring and enforcement of civil rights. Also in the 2000 Census, Hispanic was asked before race and then followed by race in an attempt to decrease the number of people (mostly Hispanic, but also Middle Eastern) who identify as Some Other Race and to get a more accurate count.
Geography
The Census provides many geographic slices of the data. Block is the smallest unit, average 100 people all within one census tract or county. Block Group is a subdivision of a census tract optimally 1,500 people and is the lowest level geography for which sample data is tabulated (confidentiality issues arise more often the smaller the population group.) Census tracts are designed to be relatively homogeneous units with an optimum size of 4,000 people. They are supposed to be stable over decades so usually follow visible features, but also include the boundary or a state or county. Places are either legally incorporated cities or Census Designated Places which are requested by the city for analysis of an area of concentrated population, not legally incorporated (downtown Amherst, for example.) Metropolitan Areas are core areas containing a large population nucleus together with adjacent communities having a high degree of economic and social integration with the core. Counties are used even if there is no county government. County Subdivisions are frequently used in New England for towns. Geographic Terms and Concepts provides a more detailed explanation.