While a traditional dryer can be operated without a vent, this is not recommended as vents are needed to remove moisture from the system. Ventless dryers that remove moisture through other means are available.
As a dryer warms clothes, water turns to water vapor. In traditional dryers, this vapor is blown from the system through the vent, and new air is pulled into the system to continue the process. Condensation dryers have a section that cools warm air, which causes the water vapor to become a liquid that is drained from the system. Heat pump dryers draw cold air from the room, which causes water to condense on the heat pump itself.