The term medicine ball first surfaced in an English-language dictionary in the late 19th century. In those days, the concepts of health and medicine were more closely related than today. Medicine balls were on the short list of exercise equipment that were considered essential.
Medicine balls typically range in weight from a couple of pounds to over 20 pounds. Nowadays they are usually made of dense rubber, though there are some with softer outer layers, and you can find certain variations like medicine balls with built-in handles or even with a rope/tether attached.
What does one do with a medicine ball? TONS of exercise moves, that's what! Remember, it's basically a weight you can toss. You can simply hold the ball while doing your regular exercises (think squats, lunges, overhead press, seated core twists...) and use it to add resistance throughout the range of motion, or you can work in some power moves like woodchoppers, medicine ball chest pass, and other fun explosive moves. I even make my personal training clients run sprints and shuffles holding them, pairing that with other moves using the ball.
Remember...the best pieces of exercise equipment aren't always shiny and expensive or "as seen on TV". Old school still works!