The ACS is an ongoing sample survey which began in 2005. It goes to about 3% of households in a given year, as compared to the decennial census long form which went to 1 in 6 households once every 10 years. It covers more topics than the decennial census. Questions asked cover:
•family and relationships
•income and benefits
•where you work and how you get there
•where you live and how much you pay for some essentials
•and other topics related to people, households, families and housing
Which estimates you depends on the size of you're geography. If you want census tracts you'll need 5 year estimates.
American Factfiner is the Census Bureau's main portal for accessing Census data. A new version of American Factfinder was released in January 2011. Data sets will be migrated from Legacy Factfinder to the new Factfinder through fall 2011. Legacy Factfinder will go dark in January 2012.
For help identifying census geographies, e.g. all census tracts in a place