Luton has had an airport for some years; in 1990 it was renamed London Luton and recognised by IATA as a London airport due to its proximity to the city.
Some believe that Luton is not properly a London airport, but for addresses in central London, travelling by public transport, the journey time to Luton will be no greater than to Stansted, Heathrow or Gatwick.
Luton has a single runway (08/26) of 7086 feet, with a terminal area to the north. Club and recreational flying are permitted.
Fast Thameslink trains run to St Pancras in central London every 15 mins, taking about 35 mins. Once every two hours, a Midland Mainline train runs non-stop to St Pancras, taking about 25 mins.
MML trains run in the other direction too, towards Derby, Nottingham and Sheffield. Thameslink trains only run as far as Bedford but you can pick up more MML intercity services from there to the north.
Thameslink usually runs through central London and south to Brighton, but at the moment a new subsurface station is being built for the line at St Pancras so they are terminating there, with a 10-15 minute walk taking you to King's Cross Thameslink for onward trains to London Bridge & Brighton.
Fares on Thameslink are about �13.40 for a One Day Travelcard (offpeak) or �10-ish for an offpeak single or day return to London.
You can pick up coaches from just outside the terminal building at the airport to a variety of destinations around the country. Green Line coaches (route 757) operate from the airport to Baker St, Marble Arch and Victoria, taking about 1h15m and currently costing �10.50 single, or �10 if you tell the driver you flew with Easyjet. You can also book National Express tickets for travel to destinations other than London at the Green Line desk in Arrivals.
Finally, for the most cost-conscious traveller, easyBus run minibuses from outside the terminal to Baker St in central London. If you book in advance on the internet then you can get a �2 single fare per seat (although if you have more than one piece of luggage you may have to book an extra seat for that).