A cleaver is a heavy, wide-bladed knife with a thick, rectangular blade, built for chopping through bone and other dense ingredients rather than fine slicing. Its weight does most of the work, driving through tough cuts of meat, root vegetables, and bones in a way a standard chef's knife isn't designed for.
The cleaver most familiar in Western kitchens is the Chinese chef's knife (cai dao), used for everything from mincing garlic and ginger to splitting whole chickens or pork bones, and it plays a similar role in Korean and Japanese kitchens for cutting through cartilage and bone. Sizes and weights vary by tradition - Chinese cleavers tend to be larger and heavier than the lighter Japanese and Korean versions - and blades are made from carbon steel, stainless steel, or Damascus steel.
Because of its weight, a cleaver takes a firmer grip and more deliberate motion to control safely than lighter knives, and it needs regular sharpening to stay effective.