The term "dryer" typically refers to clothes dryers, which uses electricity or fuel to generate heat. This heat is sent into a tumbling drum to convert water in the clothing into water vapor, which is then removed from the system.
To keep water vapor from condensing back on the clothing, traditional dryers vent air out of the system. Some newer dryers offer ventless operation. In these systems, moisture within the dryer is cooled in a special compartment where it can be drained out of the system. In most cases, gas-powered dryers cost less to run than electrical dryers, although costs vary.