Microdata is the individual-level responses to the ACS that is used to create the summary and detail tables the Census publishes. Instead of a getting one row per state from a table, we can get one row per respondent. For the American Community Survey, this data is called the Public Use Microdata Sample (PUMS).

Using PUMS data instead of the published tables can be a very powerful tool. It can, for instance, allow you to create custom estimates that aren’t available in pre-aggregated tables. You can also use microdata to fit models on individual-level data instead of modeling block groups or census tracts.