A chair is a piece of furniture often with four legs and a back and is used for sitting. If the chair lacks a back or arm rests it is called a stool.
Rex Callan took a seat on a bar stool in Sam's bar while he waited for Brendan Patol to appear. During a brawl in the bar he used the stool to ward off one of his assailants. (Star Trek: The Farthest Star: "Initiation")