A hammer is a tool you can use to drive nails into wood or other materials. You'll find a hammer in just about any toolbox, since it's useful for hanging pictures, making repairs, or breaking things apart.
When you hit a nail again and again with a hammer, you hammer it. Other kinds of hammers include the part of a gun that swings forcefully when the trigger is pulled; the padded striker in a piano that hits a string when a note is played; and a Marvel Comics supervillain whose partner in crime is named Anvil. There's also the hammerhead shark, named for the odd, hammer-like shape of its head.