Block groups consists of clusters of blocks, and are statistical divisions of census tracts. BGs generally contain between 600 and 3,000 people. Each block group is uniquely numbered within a census tract (each census tract contains at least one block group), and never cross state, county or census tract boundaries.
Below is an image of a census block group:
Block groups also contain a code range, of a single digit between 0-9. These single digits represent all blocks within the range, such that BG 4 contains all blocks 4000-4999. See the picture below for an explanation of a location in the United States:
United States Census Bureau, Geographic Terms and Concepts – Block Groups