Ventless (condensor) dryers are not ducted to the outdoors for dissipating moisture. They operate essentially as a closed system. Hot air from the drum is blown over a condensor unit, typically a set of hollow tubes. Cool room air is blown through the inside of the tubes which entices the moisture collected from the drying clothes to condense on the outside of the tubes and drip into a tray. The tray may have a pump that sends the collected water to the drain, or some have to be dumped manually. They're often installed in locations where exhausting outdoors would be a problem, such as interior apartments of large buildings.

Condensor dryers are not a new thing. Frigidaire, GE, Maytag, and Bendix had them in the 1950s and 60s. Some of them back then had the condensor cooled by a mist of cold tap-water, which was pumped to the drain along with the collected moisture. Water-cooling is rare to non-existant nowadays due to water conservation concerns.