A knapsack sprayer is a manually operated or motorized spraying device worn on the back like a backpack. It consists of a tank to hold liquid, a pump mechanism, a hose, and a spray lance with a nozzle. The user pumps or activates the sprayer to create pressure, allowing the liquid to be sprayed evenly over crops. It is widely used in farming and gardening for pest control, weed control, and fertilization.