An airport can't exist in isolation. It depends on a massive surface-transportation system so that people can get to and from the airport, park and get from place to place within the airport structure itself. While your first thought about an airport is air travel, ground transportation is pretty crucial to an aiport's operation.

The busiest airport in the world is Atlanta's Hartsfield International Airport. Here are some ways ground transportation is critical to that airport:

Roads allow access to and from the airport: In Atlanta, four interstate highways move traffic to and from Hartsfield. There's also a station for MARTA trains to connect into the city's rapid transit system.
Parking allows short- and long-term storage of automobiles. Parking can be on or off airport grounds, and some parking systems are run by private vendors under airport regulation. Hartsfield has 30,000 public parking spaces.
Passenger drop-off and pick-up areas make it easier for passengers to get into the terminals, although they are often plagued by traffic congestion because so many people are trying to get in and out.
Rental car companies serve airports. Hartsfield has eight rental car companies on airport grounds and another three off airport grounds.
Shuttle services provide passengers with transportation to local hotels and off-site parking facilities. Hartsfield is served by 18 hotel/motel shuttle buses.
Private transportation is available in the form of limousines, vans and taxis.
Public transportation (such as municipal buses and subways) may have stations at an airport. Besides the MARTA station at Hartsfield, 12 bus lines (public and private) serve the airport.
Internal subway trains and trams may be available to help passengers get to the terminal gates from the concourse. Hartsfield's People Mover is a 3.5-mile (5.6-km) loop track that has 13 stations serving six concourses with nine four-car trains; the trip is two minutes between stations.

Photo courtesy Denver International Airport
Jeppeson Terminal (middle), showing access road, passenger drop-off/pick-up areas (left) and parking garages (right)

Now that we know about ground transportation, let's move on to the core of the airport: concourses and terminals.