There are many different types of dryers: top loading, front loading, gas dryers, electric dryers, stand alone dryers, stackable units, and many others. Although there are many different types, the basic principles of how they work is very similar. They all contain a tumbler (the round holding chamber where the clothes go), a heater, a lint trap, and an exhaust vent.
For this description, we'll start from the top, just like you're doing a load of laundry.
First, you'll open the door and place your wet clothes into the tumbler. After you shut the door, you will set the heat settings. Most dryers have three settings, such as low, medium, or high heat. Next, you'll set the timer. It's usually a dial type of timer with various settings. You can set the timer for a specific length, like 20 or 80 minutes, or you can let the dryer decide when it's time to turn off. Most newer dryers have cycle settings like "optimum dry," "less dry," "fluff," and various others. Some have controls for cotton clothes or permanent press, or other options. Also included is a notification switch, that when set on "loud," sounds a buzzer to let you know the dryer's done. After all the settings are set, you press the "start" button.
The motor begins to turn and the belt rotates the tumbler. At the same time, air is drawn into the dryer. Also at the same time, the heating coil flares to life (or the pilot light ignites the heater, in a gas dryer) and warms the incoming air. The tumbling clothes heat up and the water within them turns to steam. The exhaust carries the steam out of the dryer, usually through a duct that sends it outside.
Sounds simple enough, right? But how does the dryer know not to get too hot, and how does the timer actually work?
When you open the control panel on the dryer - which is not recommended unless you know what you're doing, by the way - there will be wires going this way and that. The cycle controller is usually square and has a round device attached to the top of it. The round device is actually a small motor. The motor has a small gear on it that connects to a larger one with a dial inside the cycle controller. As the gear on the motor turns, it turns the dial very slowly. The dial usually has a set of four cams attached to it. As the dial turns, the cams engage certain contacts within the cycle controller. The contacts determine everything that happens, from the length of the drying cycle to whether or not the air gets heated.
To ensure your clothes don't go up in flames, dryers have temperature sensors installed in them. They are little silver discs, about the size of a quarter, and there are usually two of them. In most dryers, one is located near the heating coil and the other is in the front section of the dryer. They also have a set of contacts within them, so that when the temperature gets too hot, the contacts separate and the dryer automatically turns off.
If you take the top and front of the dryer off (again, not recommended), you will see the exposed tumbler. A belt goes around the tumbler and winds around a pulley that's connected to a motor. There are rubber wheels underneath that sit in a groove in the tumbler to ensure an even and smooth rotation. Depending on the dryer you have, the motor may control the exhaust fan as well.
One of the most important factors in how a dryer works is air flow. The dryer has to bring in air to heat in order for everything to work. Most dryers have a hole or ventilation duct located on the front of the machine. It's not visible with the cover on. Air is sucked in through that hole and is forced past the heating element. The heated air flows into the tumbler to heat the clothes. The steam is then sucked out of the dryer through the lint trap and out of the building. The lint trap is usually located right below the door of the dryer. For your dryer to work as efficiently as possible, make sure to clean the lint trap after every load. A fan is used to both bring the air in and send it out. It is located between the lint trap and the ductwork leading outside.

For all intents and purposes, the dryer is a very simplistic machine. It is designed so well, that the only things that really change over the years is the look of them. But just because they're designed well doesn't mean you'll never have a problem with them.

What the temperature thermostat is, and how it works:

The temperature of the air inside the drum is regulated using different thermostats. Thermostats open and close an internal switch when subjected to the temperature of their environment.
There are two different thermostats in your dryer, the operating thermostat(s) and the high limit ( safety ) thermostat(s). The only difference in these thermostats is their opening and closing temperatures.
Some common area's to find the thermostats. The operating thermostats are often found on the fan housing on Maytag, Whirlpool and Kenmore dryers. Operating thermostats for an older GE dryer are just under the lint filter area, the newer GE dryers have them on the fan blower at the front.
Safety thermostats are usually always found on the heating element housing or cage assembly.

Nine money saving tips that will extend the life of your dryer and lower the cost of energy. 
 1. Make sure that the washing machine’s feet are level to prevent an imbalance in the machine that may cause loud noises and added wear/tear on the unit. For fire safety, do not place the dryer near heat producing device.
2. To avoid burning out the compressor, always plug the dryer directly into the properly grounded outlet on your wall. If you absolutely need to run an extension cord, make sure a heavy duty, 3-pronged extensions cord. If unsure, consult the manufacturer specifications in the user guide.
3. Empty the lint filter after each and every use for efficient drying and fire prevention. Remove anything else that can impede good airflow. Do not forget to clean the back of the dryer where lint settles over time.
4. Vent your dryer to the outside, never vent it inside. Clean out exhaust vent system at least once a year to remove clogs or other obstructions that can impede good airflow. The U.S. Consumer Product Safety Commission (CPSC) estimates that fires associated with clothes dryers occur annually.
5. Do not place foam backed rugs, rubber/plastic shoes or other heat sensitive articles in dryers. Also, to prevent fires, do not place clothing or other items soiled with flammable substances like gasoline and alcohol. Check the pockets of clothes for crayons and other heat sensitive items that may ruin the other clothes.
6. To save energy, do not overload the dryer and make sure items are not water logged. This may burn out the motor.
7. Clean up all spills from the top of the dryer. Many detergents and solvents can damage finish. Clean with a mild household dish soap or baking soda and rinse with warm water. Rinse and wipe dry.
8. Always read manufacturer’s guides and operating instructions.
By following these tips, you can prolong the life of your dryer and avoid many costly repairs. However, there are times when a repair is needed; We fix most brands and models of dryers including Kenmore, Whirlpool, General Electric, Amana, Maytag, Jenn-Air, Roper.

How to install the dryer:

1. Allow at least 4 inches of clearance to vent out the back of the dryer. This will allow for the offset of the venting elbow. If you vent off the top of your dryer, cut out the vent hole by drilling a hole with a drill and cutting it to the proper size with tin snips.
2. The method you choose to exhaust your dryer depends on many different factors. Choose the most efficient route from your dryer to the outside.
3. Use a hinged damper weather hood to prevent backdraft and place it at least 12 inches off the ground. Be sure the placement of this vent will not exhaust dryer air to a window well, gas vent, chimney or any other unventilated area (such as attic or crawl space). The accumulation of lint can be a fire hazard so check your local safety code on this issue.
4. Connect the exhaust system with rigid aluminum pipe and wrap joints with duct tape. Avoid using sheet metal screws. Hang the aluminum pipe from appropriate hangers on horizontal lines at the critical distances determined by your local building code.

The most important maintenance for any dryer is a clear exhaust.
Many dryer failures usually result from an improperly maintained vent system. This causes premature failure of the bearings, elements, ignitors and other critical parts.
- For best drying results, clean the lint filter before each load.
- When drying only one or two items, toss in a few items of varying size to promote good tumbling action and improve drying results. Dryers with an Auto-Dry cycle will not dry properly with a very small load in the tumbler.
- A drying rack accessory sits stationary on the front and rear bulkheads to speed the drying process of items like: caps, sneakers, sweaters, stuffed toys and pillows. Ask your dealer for details.
- Raising the dryer 6-12 inches with a storage drawer below puts the door opening near level with the washer opening and reduces bending.
- Never use white plastic exhaust duct on any dryer!! The stuff is not only flammable, but it increases drying time tremendously! Do not use the foil-looking stuff, either! Use only flexible metal duct or rigid duct. The new periscope vent kits also work well for dryers in tight spaces and will eliminate any chance of hose kinking.
Improper venting or a clogged vent will give the following results: