Most suspension forks are either air sprung or coil sprung. The stiffness of the suspension is adjusted on the air sprung forks by adding or releasing air from the air valve. On the coil sprung forks, the stiffness is adjusted by turning the preload knob.
To keep the spring of the forks in control and not bouncing up and down like a pogo stick, they use a damping system. All of the dampers that I have seen are oil dampers. There are two different types of oil dampers, open damper and closed damper. Open dampers use the same volume of oil to lube the slider bushings and stanchions and to flow through the damper to control the compression amd rebound speed of the forks. The closed damper is sealed so that the damper oil is separate from the lubricant used on the slider bushings and stanchions. A closed damper is sealed in the fork and requires less service than the open damper which can become contaminated from any dirt that makes its way past the dust wiper and main oil seal at the top of the slider leg. But when a seal splits or wears out on a closed damper, it is not immediately obvious why the damping ability has been lost. On the open damper, you can see the seals and look for seeping oil without disassembling the fork.
The most important preventive maintenance for a fork, especially an open damper fork, is keeping it clean. Manufacturer's recommendations vary, but most agree that you should inspect your forks for signs of stress and oil weeping before every ride. Generally you should replace the oil and seals after every 50 hours of riding in good conditions, or 25 hours if riding in wet muddy conditions. A complete overhaul including replacement of the seals, wipers, and slider bushings is a good idea after every 200 hours of riding. Of course oil leaks should be repaired as soon as possible before performance suffers.
If you don't want to mess with a fork, consider getting a sprung seatpost. They give you extra comfort without oil filled dampers and all of the maintenance issues that accompany them.
If you still want to go with the suspension fork, the best thing to do is to consult with the sales people at your local Trek store. Trek does make several bikes that have a suspension fork, like the Navigator. They would be the best qualified to help you aquire the fork that you want that will fit your bicycle.
