Chicago is composed of districts, community areas and neighborhoods.
Districts contain community areas.
Community areas contain neighborhoods.
There are 77 community areas which were defined by sociologists from the University of Chicago in the 1920's. These community areas are still used today for the purposes of summarizing urban characteristics.
The neighborhoods, however, are dynamic - not static - and depending on whom you ask there are anywhere from "around 100" to over 230 neighborhoods. Many people use the term "neighborhood" to refer to community areas.