The flag of Chicago has broad blue stripes and four red
six-pointed stars. Originally the flag had two stars when it
was adopted in 1917. The white stripes represent the
North, West and South sides of the city. The two blue
stripes represent the two branches of the Chicago River.
The symbolism of the stars is complex. Here is how it is
described:

The first red star represents Fort Dearborn (added in 1939).
    The points of the first red stars signify:
      1. transportation
      2. labor
      3. commerce
      4. finance
      5. populousness
      6. salubrity

The second red star represents the Great Chicago Fire of October 8-10, 1871.
    The points of the second red star signify:
      1. religion
      2. education
      3. esthetics
      4. justice
      5. beneficence
      6. civic pride

The third red star represents the World's Columbian Exposition of 1893.
    The points of the third red star signify history of the area:
      1. France 1693
      2. Great Britain 1763
      3. Virginia 1778
      4. Northwest Territory 1798
      5. Indian Territory 1802
      6. Illinois Statehood 1818

The fourth red star represents the Century of Progress of 1933 (added in 1933).
    The points of the fourth red star signify:
      1. World's Third Largest City
      2. City's Latin Motto
      3. I Will Motto
      4. Great Central Market
      5. Wonder City
      6. Convention City