Historical version 3 of Gatwick Airport (view current version)
The second biggest of London's airports (after Heathrow Airport), though not really in London (it's out to the south in the commuter belt), Gatwick has two terminals (the north is primarily for long-haul flights, the south for European and charter flights) at the eastern end of the two runways (08R/26L at 10364 feet, and 08L/26R at 8415 feet, which is used mostly for lighter aircraft).
Gatwick is not on the Tube, but has its own station; trains run from Victoria Station or London Bridge Station, taking about half an hour. From Victoria, you can either take the Gatwick Express or a local train; the local train takes about five minutes longer, doesn't have extra luggage space, but costs about 1/3 to 1/2 as much. Since many airlines allow you to check in and deposit your hold baggage at Victoria Station, the local train is almost always the best option.
I have found Gatwick a remarkably unpleasant airport to pass through; whether this is because most of the package-holiday and charter flight traffic from the south of England passes through it, I don't know, but it always seems to have an unreasonably high proportion of panicking people.