They are the same thing.
Potentiometer is the name given to the smaller scaled devices such as volume controls, Rheostat is more often used when it is a larger device that handles significant power like motor control or industrial lighting controls. Most potentiometers use carbon tracks, rheostats tend to use resistance wire, sometimes wound on a heat sink or even water cooled. There is no real border between them and the construction of each overlaps to some extent.
Brian