The Census Bureau has various small area data. These are census tracts, block groups, blocks, and zip codes.
Blocks are the smallest unit.
Block groups are made up of blocks and census tracts are made up of block groups. The long form was sent out to approximately one in six households. The number of responses are then multiplied to extrapolate the data for the entire population. Census tract and block group data is available from questions asked in the long form. Amercan FactFinder is the census site for finding data from these small areas.