A chair is a seat with four legs and a back to lean against. When you invite a large group for dinner, you have to make sure you find chairs for all of your guests.
As opposed to a bench, couch, or sofa, a chair usually seats one single person. You might have wooden kitchen chairs and a comfortable, upholstered chair with arms for watching TV. Another kind of chair is the leader of an organization or group, and it's also a verb meaning to lead such a group: "I've been asked to chair the monthly meetings of the stamp collectors' club." The Old French root is chaiere, "seat or throne."