Rheostat switches are electrical switches that contain a variable resistor, which allows them to finely adjust the amount of current passed along the attached circuit. Rheostat switches were once commonly used as dimmers for electric lights.
A switch is any device that can interrupt the flow of current along a circuit. Switches can be in an "on" or "off" state. In most switches, the "on" state corresponds to current flowing along at full capacity. With a rheostat switch, once the switch is turned on, resistance to the current can be adjusted, which increases or decreases its flow.
Rheostat switches were once a common feature in dimmers, heaters, ovens and motor speed controls, but because they work by converting electricity to heat, they are not very energy efficient. As of 2015, they are still used in devices that require high voltage and current.