The word potentiometer is often used to categorize all variable resistors, but another type of variable resistor, known as a rheostat, is different from a true potentiometer. Rheostats are two-terminal devices, with one lead connected to the wiper and the other lead connected to one end of the resistance track.
Although a potentiometer is a three-terminal device — its leads connect to the wiper and to both ends of the resistance track — you can use a potentiometer as a rheostat (as is quite common) by connecting only two of its leads, or you can connect all three leads in your circuit — and get both a fixed and variable resistor for the price of one!
Rheostats typically handle higher levels of voltage and current than potentiometers. This makes them ideal for industrial applications, such as controlling the speed of electric motors in large machines. However, rheostats have largely been replaced by circuits that use semiconductor devices, which consume much less power.