Census tracts are "relatively permanent statistical subdivisions of a county…population size between 1,200 and 8,000 people, with an optimum size of 4,000 people":
Block groups are “generally defined to contain between 600 and 3,000 people”:
Census blocks are the smallest census geographies. In some cases, they have no population at all:
Census and ACS data are tabulated for a range of geographies, known as the summary level. For example, if we make a table of the total population for each county in Pennsylvania, the summary level is by county. This protects the confidentiality of residents.
Census geographies can be legal, statistical or administrative areas. Legal areas are political entities that exist outside of the census, such as states, counties, or cities. States and counties are almost always the same from year to year, while city boundaries may change. Statistical areas exist solely for tabulating data; they include census tracts, block groups, and blocks. Statistical areas are revised every ten years to reflect changes in the population. Administrative areas exist to deliver services; examples include school districts, voting districts, or ZIP codes.