Most dehumidifiers also have a humidistat, allowing you to set your desired level of relative humidity. This has two parts; a sensing element and a relay. The sensing element has metal conductors measuring the change in humidity based on the conductivity between the two parts. The relay measures this resistance in turn and decides if the dehumidifier should be running or not.
Dehumidifiers usually have a built-in reservoir to collect the condensed water. On anything other than highly portable devices there is a point to connect a hose as a permanent feed. This saves having to keep draining the reservoir and is useful for situations where the dehumidifier is left unattended for long periods of time.