Think about an ordinary motorcycle: you have a heavy engine in the center with the rider balanced on top of it and two narrow wheels with rubber tires just in front and behind. Now in theory you can ride a machine like this through snow, providing the snow is soft enough to compact as you move over it (and you can get some grip) and not so hard and frozen that it's turned to super-slippery ice. In practice, motorcycling on snow and ice is incredibly dangerous and best avoided: steering fluctuates between tricky and impossible and there's a high risk your bike will slide right out from under you. The problem is that relatively little of your bike is actually touching the road—just two tiny patches of rubber under the front and rear tires—and that's simply not enough to give you good traction and grip.

If you had to rebuild a motorcycle for snowy terrain, what would you change? You might fit skis at the front to spread your weight and give reasonable steering. A wide, rutted track at the back would give you plenty of speed and lots of grip, no matter what kind of terrain you had to deal with. What you'd end up with would be something like a modern snowmobile. It's very similar to a motorcycle, with the engine in roughly the same place and power transmitted to the rear track through a drive belt.
Artwork: A snowmobile gets much more traction in snow than a motorcycle using a large, wide track with deep rutted treads. A belt-drive and clutch system transfers power from the engine (in the center of the machine) to the track at the back. Compare this with Bombardier's early, 1944 snowmobile design (below) and you'll see that the basic idea hasn't changed very much.

What are the parts of a snowmobile?

There's lots more to a snowmobile than just an engine, some skis, and a track. Here are a few of the more obvious bits and pieces:

Skis: These curve upward to stop the machine diving into the snow as it plows forward. They also provide effective steering.
Ski handles: These are extra loops of metal on the front of the skis, used for towing or pulling the machine free of snow.
Front bumper: A bit of crash protection!
Shock absorbers: Often made from lightweight but tough aluminum, they absorb energy as the skis hit bumps.
Hood: Fiberglass or tough polycarbonate plastic front cover. Conceals energy absorbing foam to soften impacts.
Engine air vent: Remember there's a gasoline engine inside a snowmobile that needs air to burn fuel. There are usually air vents on the front and sides of the hood.
Dashboard instruments: The latest snowmobiles have LCD display panels including a speedometer (showing current speed and average speed), rev counter, mileometer, engine thermometer, and compass.
Windshield: It measures ~12–60cm (~5–25 inches) from top to bottom. High windshields like this one are good for exploring tricky terrain and provide a certain amount of anti-roll protection; lower windshields are better for racing.
Side reflector: A bit of added visibility!

Foot rest: Somewhere to rest your feet, of course, but also protection from the icy wind.
Throttle (accelerator): Expect a top speed of about 110km/h (70mph).
Brake lever: operates hydraulic disk brakes that stop the rear tracks. Obviously the front skis have no brakes on them!
Seat: There's room for one or two people on most snowmobiles. Some models have heated seat covers.
Idler wheels: These don't provide power: they allow the track to spread over a larger area of the ground, effectively giving a longer track and providing better traction. They also provide suspension. Idler wheels are about 14cm (5.5 inches) in diameter. The idler wheels are the ones colored orange in our simplified cutaway drawing up above.
Track: Sometimes known as the "endless tread." Typically 34–40cm (13.5–16 inches) wide and made from tough materials such as carbon fiber or Kevlar.
Sprocket wheel: The back wheel is typically about 18cm (7 inches) across.
Snow flap: Like the mudflap on a bicycle, this stops snow shooting up from the spinning track at people following on behind!
Luggage rack: Most snowmobiles carry a fuel can here or somewhere on the body.
Tail/brake light.
Seat back rest.

Front headlight: Fitted with one or two powerful 60-watt bulbs with lenses to concentrate the beam.
Rear bumper.
Skid plate: For bumping through snowdrifts.
Wing mirror.
Fuel tank: Holds ~40liters (10.5 gallons) and gives a range of ~300km (~200 miles). Typical fuel consumption for snowmobiles is about 10-20 liters/100 km (10-20 mpg).
Handlebar wind deflectors: Help protect your hands from freezing up in the blast of a chill wind. Snowmobile handlebars generally have heated grips for the same reason.
Engine: Usually a 2-stroke 600cc or 4-stroke 1200cc engine (similar to a medium-to-large motorcycle). Power is transmitted from the engine to the rear track system by a simple drive belt, as shown in our little artwork up above.
Rear shock absorber and suspension.

Who invented the snowmobile?

Think "snowmobile" and you probably think "Ski-Doo" and "Bombardier," but invention tends to be more of a process than a sudden event—and, as with other inventions like cars and computers, quite a few other people tried their hand at creating mechanized snow craft.
Back in 1908, Alvin Lombard made a motorized steam tractor on skis, which he named the Lombard Log Hauler.

In 1913, Edward L. Schuh was granted a patent for this "runner attachment for automobiles whereby the vehicle may be utilized in winter as well as in summer and may be propelled under its own power with out burning out tires or any excessive strain on the engine."
Other handy inventors also experimented with turning cars into snowcraft, including a New Hampshire Ford dealer named Virgil T. White (who came up with the name "snowmobile")—and Joseph-Armand Bombardier himself, a Canadian who reputedly began tinkering with snowmobiles when he was just 15. It was 1959 when he built his Ski-Doo, which was the first, real, commercially successful snowmobile.