A census block is the smallest geographic unit used by the United States Census Bureau for tabulation of 100-percent data (data collected from all houses, rather than a sample of houses). Several blocks make up block groups, which again make up census tracts. There are on average about 39 blocks per block group, but there are variations. Blocks typically have a four-digit number, where the first number indicates which block group the block is in. For example, census block 3019 would be in block group 3. The number of blocks in the United States, including Puerto Rico, is about 8,200,000.
Blocks are typically bounded by streets, roads or creeks. In cities, a census block may correspond to a city block, but in rural areas where there are fewer roads, blocks may be limited by other features. The population of a census block varies greatly. There are about 2,700,000 blocks with a population of 0, while a block that is entirely occupied by an apartment complex might have several hundred inhabitants.
- Census Bureau Definition of Block Group - http://www.census.gov/geo/reference/gtc/gtc_bg.html
- Some statistics can be found in the Census Bureau's description of LandView 6 software.