Census data are available for a number of different geographic units.
Community areas. Community areas, of which there are 77 in Chicago, are supposed to be homogenous neighborhood-like districts. Many, but not all, do in fact correspond quite closely to neighborhoods that would be recognized by their residents; examples of these are Hyde Park and Uptown. Community areas are peculiar to Chicago. They were apparently first proposed by members of the Social Science Research Committee at the University of Chicago during the 1920s. They have been used ever since by the city government as statistical units. They have the virtue of consistency; except for the addition of O'Hare and the splitting of Edgewater from Uptown, they have remained unchanged since their inception. The U.S. Census Bureau does not compile data for community areas. Although most community area boundaries are coincident with census tract boundaries, because the city limits intersect census tract boundaries in several places, it is not possible to derive community area data from census tract data in a straightforward way.
Census tracts. Census tracts are small areas that are supposed to be somewhat homogeneous. Census tracts ideally have something like 1200 households (perhaps 2000-4000 people), but, in Chicago, population varies from 0 up to 10,000. Census tracts in the city of Chicago have remained nearly constant since the 1920s, but the numbering system has changed. There are ca. 866 census tracts in Chicago. Census tracts in the suburbs have changed a great deal over the years, in most cases by splitting.
Census blocks. Census blocks correspond closely to blocks that any urban resident would identify. Only limited data are available at the block level, and some figures that one might think would be available are suppressed to prevent identification of individuals. Census-to-census comparison is difficult because of changing systems of block identification. There are something like 10,000 census blocks in Chicago.
Census data are available for other geographic units as well. Block groups consist of several blocks; there are typically between three and five block groups to a tract. All the variables enumerated at the tract level are, in theory, enumerated at the block group level too, but data suppression is more common, and census-to-census comparison is difficult. Census data are also available for zip codes, congressional districts, and (outside of Chicago proper) places (e.g., cities), and minor civil divisions (i.e., townships). The City of Chicago (but not the Census Bureau) has occasionally released demographic data for wards.
Sources of Historical Data
The Census of Population and Housing [HA 201 year] contains original census data, published in many volumes. Most volumes have appropriate maps at the end. Often there are second copies of the maps at the Map Collection [G4104.C6E25 year].
The Local Community Fact Books contain community area data. They were published approximately every ten years between 1938 and 1995 [F548.68.A1C5, F548.68.A1H38, F548.68.A1H38 1960, F548.68.A1L58, F548.68.A1L58 1995 (some copies in reading rooms)]. See also Census Data of the City of Chicago, 1930, Community Areas [HA730.C5C85].
The Map Collection holds hundreds of paper maps that were compiled on the basis of census data. Most are cataloged. Records can perhaps be found most easily by doing "Anywhere keyword" searches on such terms as "Chicago," "maps," and "census." If you understand how map call numbers work, you can also search productively by call number (try, for example, G4104.C6E1 or G4104.C6E73).
Census Data of the City of Chicago, 1920 [HA730.C5B9]. Census Data of the City of Chicago, 1930 [HA730.C5B91], and Census Data for the City of Chicago, 1934 [HA730.C5N5] have accurate titles. Books contain both data and maps.
The National Historical Geographical Information System at the University of Minnesota has both ArcView shapefiles with tract boundaries and dbf files with tract-level data back to 1910 that you can download.
The Center for Population Economics at the University of Chicago has ArcView shapefiles with ward boundaries back to the 19th century that you can download.
Sources of Relatively Recent Data
American Factfinder allows you to download 2000 and 2010 census data.
The ESRI Website allows you to download ArcView/ArcGIS shapefiles for census tracts and other census units for 1990 and 2000.
The Map Collection Website has some non-customizable 1990 and 2000 census maps for the Chicago area. Most are choropleth maps, but there are also some dot maps showing population change.
Several discs from GeoLytics, held at the Map Collection, allow you to download census data and census boundary files for 1970, 1980, 1990, 2000, and 2010. One disk (the Neighborhood Change Data Base) has 1970, 1980, and 1990 data projected to 2000 boundaries. Another GeoLytics disk has a much larger set of 1990 data projected to 2000 boundaries. The GeoLytics disks may only be used at the Map Collection; you are of course free to download and take away data.
At the Map Collection you can use ArcView or ArcGIS to make customized maps based on census data. You can also combine census data with other spatial data. Staff are available to help.