The main US Census Bureau site is at http://www.census.gov.
When using Census information in GIS, you need to have both geographic boundary files (e.g., census tracts) and the census data (attribute tables) that join to those boundary files.
Please follow these instructions to download 2010 and 2000 data from the census: Census Tutorial: Downloading and Mapping American FactFinder Data.
Data available depend on the year. After 2000, estimates are collected and published depending on the how big the population of area is. Please see American Community Survey for estimate schedules. For the years 2000 and 1990, data typically come from either the 100% census (SF1 and SF2) or from the sample “long form” sent one out of every several households (SF3 and SF4). The 100% SF1 data has basic demographic information on population counts, age, gender, race, ethnicity, and household/family relationships. The sample SF3 data contains a wealth of more detailed information including income, employment, education, transportation to work, language, and ancestry.
- Cartographic boundary files from the 2010 TIGER/Line Shapefiles Main Page . This is a good place to get the Zip Code Tabulation Area boundaries, but make sure you read about them first – ZCTAs are not the same thing as Zip Codes. See this ZCTA description site from the Census Bureau.
- National Historic GIS Project is a project to digitize and distribute aggregated census data for US Censuses 1790-2000. Also included are boundary GIS shape files for previous censuses. An invaluable site for historic research in US. Easy to use interface.
- IPUMS USA Integrated Public Use Microdata (Minnesota Population Center, University of Minnesota) is very good site with census samples from different years, information about previous censuses, including enumeration forms ( the questions asked) and lists of variables for all US censuses. Also has an international section.
- ICPSR Historical, Demographic, Economic, and Social Data: The United States, 1790-1970. Tufts University (and many other university researchers) can download historical US census data from this site. Note this site is for experienced users. See the university reference librarian for assistance. To view data from this series, see the Historic Census Browser from the University of Virgnia described above.