Public transportation from the airports and within the City of Chicago is covered by the CTA. In particular
there are the following maps of the system of "L" trains:

• For the entire system
• For the city center (including Navy Pier)

From O'Hare Airport to the City you could take the blue line to Washington Station and
switch there to the red line north to Grand which brings you within a few blocks of Navy Pier.

The CTA website also has maps and schedules for the bus lines.

The suburbs and the greater metropolitan area is covered by METRA trains.