The Municipal Flag of Chicago

The Municipal Flag of Chicago consists of 3 White stripes separated by two stripes of Blue with four Red six-pointed stars on the center stripe of White...

The 3 White stripes:
Top White Stripe represents the North side of the city.
Center White Stripe represents the West side of the city.
Bottom White Stripe represents the South side of the city.

The 2 Blue stripes:
Top Blue stripe represents Lake Michigan and the North Branch of the Chicago River.
Bottom Blue stripe represents the South Branch of the Chicago River and the Great Canal.

The 4 Red stars on the Center White Stripes represent something as well as each of the points on the stars themselves:
The First Red Star represents Fort Dearborn (added in 1939).
The Second Red Star represents the Chicago Fire of October 8-10, 1871.
The Third Red Star represents the World's Columbian Exposition of 1893.
The Fourth Red Star represents the Century of Progress Exposition of 1933 (added in 1933).

reference: http://www.chipublib.org/004chicago/flagtxt.html