AbbyNormal,
See below a description from wikipedia, better than I would've described it.
I guess, a better description should be "condensing and evapoprating", condensation being only the first step prior to evaporation.
Ventless dryers
Spin dryers
These machines simply spin their drums faster than a typical washer could in order to extract additional water from the load. They may remove more water in two minutes than a heated tumbler dryer can in twenty, thus saving significant amounts of time and energy. Although spinning alone will not completely dry clothing, this additional step saves a worthwhile amount of time and energy for large laundry operations such as those of hospitals or hotels.
Condensation dryers
Just as in a normal dryer, condensation dryers pass heated air through the load. However, instead of exhausting this air, the dryer uses a heat exchanger to cool the air and condense the water vapor into either a drain pipe or a collection tank. Afterwards, this air is run through the loop again. The heat exchanger uses ambient air as its coolant, therefore the heat produced by the dryer will go into the immediate surroundings instead of the outside.
Because the heat exchange process simply cools the internal air using ambient air, it will not dry the air in the internal loop to as low a level of humidity as the fresh, ambient air. As a consequence of the increased humidity of the air used to dry the load as well as the increased complexity of the design, this type of dryer requires more time and energy than a traditional dryer. However, it is a valid option where long, intricate ducting would be required to vent a traditional dryer.
Heat pump dryers
Whereas condensation dryers use a passive heat exchanger cooled by ambient air, these dryers use a heat pump. The hot, humid air from the tumbler is passed through a heat pump where the cold side condenses the water vapor into either a drain pipe or a collection tank and the hot side reheats the air. In this way not only does the dryer avoid the need for ducting, but it also conserves much of its heat within the dryer instead of exhausting it into the surroundings. Heat pump dryers can therefore use less than half the energy required by either condensation or traditional dryers.
As with condensation dryers, the heat exchanger will not dry the internal air to as low a level of humidity as the ambient air. With respect to ambient air, the higher humidity of the air used to dry the clothes has the effect of increasing drying times; however, because heat pump dryers conserve much of the heat of the air they use, the already-hot air can be cycled more quickly, possibly leading to shorter drying times than traditional dryers, depending on the model.
