A hammer is a tool for putting nails into objects such as wood. It has a long handle, and a tip with a flat end for hitting nails. The most common type of hammer (sometimes called a claw hammer) has a curved end (on the other side of the hitting end) for pulling nails out.
A sledgehammer is a hammer that has a longer shaft, and a more massive head. The user can swing it with two hands and apply more force.