A steam dryer is a conventional dryer with a steam feature. The steam feature will create steam in the dryer independently of the traditional drying cycle. This feature is designed to refresh clothes that might not need to be washed, or to remove wrinkles from a load of laundry.
A steam dryer can work in a number of different ways. For example, if you have an article of clothing with a slight odor, or it has just been worn once and you want to wear it again, you can toss it into a steam dryer for what is usually a 20-minute or less cycle. The dryer will then produce warm steam to remove odors and wrinkles from the clothing.
The other way a steam dryer works is at the end of a drying cycle. Clothes dryers work by heating the air around the circulating clothes, which can easily cause wrinkles when the load stops moving. If your dryer has a steam feature, the steam might start at the end of the drying cycle to prevent wrinkles from setting in. Additionally, if you've accidentally left clothing in the dryer for a few hours, or overnight, and they have gotten very wrinkled, running the steam dryer feature for a few minutes is a quick and easy way to remove wrinkles.
Dryers with a steam feature are usually considerably more expensive, sometimes by as much as twice that of standard dryers. Not all owners believe that the extra cost is worth it. The same steam effect can be achieved by placing a wet item in the dryer with a dry, wrinkled item. As the wet item dries, it will release steam, which will again refresh and de-wrinkle the dry item. Keep in mind that with some steam dryers, it is not possible to steam an entire load of clothes at once. This could potentially use more energy and raise your electric or gas bill if you need to divide up your loads of clothing to steam them.
Steam washers are often sold as a set with steam dryers. Steam washers work in a similar way, and are often more effective at removing dirt, allergens, and detergent from clothing. These extra features are not something that steam dryers are capable of doing, as dryers simply remove odors and wrinkles. Keep in mind, however, that though it may be tempting, there is no reason you need to buy a matching washer and dryer.