Hammer

A hammer is a tool meant to deliver blows to a target, causing it to move or deform. The most common uses are for driving nails, fitting parts, and breaking up objects. Hammers are often designed for a specific purpose, and so their design varies quite a lot. Usual features are a handle and a head, with the balance firmly in the head. The hammer is used in many professions, and is one of the most basic tools along with the knife. 

Like the knife (and almost all tools), the hammer is also a weapon. The concept of putting a handle on a weight to make it more convenient to use may well have led to the very first tools or weapons ever invented. 

The use of a hammer to fix broken machinery is jokingly referred to as percussive maintenance. 

Well-known forms include: 

Carpenter's hammer, including framing hammer and claw hammer 
Mechanic's hammer, including ball-peen hammer 
Construction hammer, including sledgehammer 
Stonemason's or club hammer 
Steam hammer 
Mallet