The ACS pretabulated (or summary) data are predefined tabulations of characteristics created by the Census Bureau in response to data user needs. The basic unit of analysis is a specific geographic entity -- state, county, etc. -- for which estimates of persons, families, households, or housing units in particular categories are provided.
With microdata, conversely, it is the user who determines the structure of the tabulation and the characteristic(s) to be tabulated.
In the ACS microdata, the basic unit is an individual housing unit, persons who live in the selected housing unit, or a group quarters person. Each record shows most of the information associated with a specific housing unit or individual except for personal identifying information and some things that could be used to identify an individual. The most detailed geographic area available in PUMS are Public Use Microdata Areas (PUMAs), or areas containing about 100,000 residents.