One thing that confuses at O'Hare is that Terminal 3 is for American flights (and Terminal 1 for United), but the only customs are in Terminal 5, so all incoming international passengers go through there. For AA/UA inbound passengers this can be unexpected. The inbound aircraft will either dock there and be tugged across to its relevant departure terminal later in the day, or you will be bussed over from the aircraft.
Although there are check-ins for all overseas airlines, there is no check-in (or departing flights) for AA/UA in T5, you need to go to the airline's normal terminal for all departures, so this transfer doesn't apply on your return journey.
If being met off an incoming AA/UA international flight there's a long tradition of greeters going to the wrong terminal. They're quite a way apart.
One facility the two main US airlines do maintain in T5 is a bag drop immediately beyond exiting from customs, where you can deposit your bags and thus be unencumbered going over to your departure terminal. When things work well (which is of course most of the time) this works very efficiently and they handle bags for short connections nicely, but if things go awry the opposite can be true.
The rubber-tyred train (no charge) between terminals is known as the Airport Transit System (ATS) which is a simple straight line which links the terminals (numbered 1-5) and the parking lots (lettered A-G). It has its own Wikipedia entry
Airport Transit System - Wikipedia, the free encyclopedia
As in many US airports a certain familiarity seems assumed, along with inconsistent signing (which might refer to ATS one moment, Terminal Shuttle the next, etc).