A rheostat is a variable resistor that is able to control current (resistance) in a circuit. It can lower or raise the flow of current in circuit without any interruption. The way rheostats are built is very similar to potentiometers their construction is reminiscent of potentiometers. For the most part rheostats have two terminals with one being connected the resistive track and the other to the wiper that moves along the resistive track to lower or raise the current output. However the big difference is that rheostats carry a significantly larger current load than potentiometers. That is why the majority of rheostats are built as wire wound resistors where the restive track is made out of an insulated ceramic core wound with a restive wire.

Rheostat Symbol
IEC Standard

Rheostats are most commonly used as devices to control power output. They can be used to control the speed of a motor, light intensity (dimmer), and the heat output of a heater/oven.