A jacket is a lightweight or cropped coat. Most jackets end at your waist or hips, while coats can be much longer. If you have outerwear made of denim or leather, feel free to call them jackets.
Jackets are generally long-sleeved and fasten with zippers, snaps, or buttons down the front. Other meanings of jacket generally include the sense of "outer covering," like a book jacket or the insulating jacket around your house's pipes. The word comes from the Middle French jaquet, "short coat with sleeves," from the Old French jacque, "tunic."