Everyone brings one piece of the plane along when they come to the airport. They all go out on the runway and put the plane together piece by piece, arguing non-stop about what kind of plane they are supposed to be building.
Everybody pushes the airplane until it glides, then they jump on and let the plane coast until it hits the ground again. Then they push again, jump on again, and so on...
All the stewards, captains, baggage handlers, and ticket agents look and act exactly the same. Every time you ask questions about details, you are gently but firmly told that you don't need to know, don't want to know, and everything will be done for you without your ever having to know, so just shut up.
The terminal is pretty and colourful, with friendly stewards, easy baggage check and boarding, and a smooth take-off. After about 10 minutes in the air, the plane explodes with no warning whatsoever.
Windows NT Air
Just like Windows Air, but costs more, uses much bigger planes, and takes out all the other aircraft within a 40-mile radius when it explodes.
Windows XP Air
You turn up at the airport,which is under contract to only allow XP Air planes. All the aircraft are identical, brightly coloured and three times as big as they need to be. The signs are huge and all point the same way. Whichever way you go, someone pops up dressed in a cloak and pointed hat insisting you follow him. Your luggage and clothes are taken off you and replaced with an XP Air suit and suitcase identical to everyone around you as this is included in the exorbitant ticket cost. The aircraft will not take off until you have signed a contract. The inflight entertainment promised turns out to be the same Mickey Mouse cartoon repeated over and over again. You have to phone your travel agent before you can have a meal or drink. You are searched regularly throughout the flight. If you go to the toilet twice or more you get charged for a new ticket. No matter what destination you booked you will always end up crash landing at Whistler in Canada.
Disgruntled employees of all the other OS airlines decide to start their own airline. They build the planes, ticket counters, and pave the runways themselves. They charge a small fee to cover the cost of printing the ticket, but you can also download and print the ticket yourself.
When you board the plane, you are given a seat, four bolts, a wrench and a copy of the seat-HOWTO.html. Once settled, the fully adjustable seat is very comfortable, the plane leaves and arrives on time without a single problem, the in-flight meal is wonderful. You try to tell customers of the other airlines about the great trip, but all they can say is, "You had to do what with the seat?"
Sorry, this is the best analogy on the subject of linux, has been around for a long time, and is anonymous.
BTW, linux is the kernel, GNU/GPL software make up the rest of the OS and apps. GNU/Linux is the way lawyers will say it in court.