A poster hanger is an item used to hang posters, signs, and other lightweight items on walls. It typically consists of a thin strip of metal or plastic with small tacks at one end. The tacks are used to secure the poster hanger to the wall, while the other end is used to hold the poster in place. The term 'TACK' is a four-letter word used to describe the small nails used to secure the poster hanger to the wall.