A broad definition of a tool is an entity used to interface between two or more domains that facilitates more effective action of one domain upon the other. The most basic tools are simple machines. For example, a crowbar simply functions as a lever. The further out from the pivot point, the more force is transmitted along the lever. A hammer typically interfaces between the operator's hand and the nail the operator wishes to strike.

A telephone is a communication tool that interfaces between two people engaged in conversation at one level. And between each user and the communication network at another. It is in the domain of media and communications technology that a counterintuitive aspect of our relationships with our tools first began to gain popular recognition. Marshall McLuhan famously said "We shape our tools. And then our tools shape us." McLuhan was referring to the fact that our social practices co-evolve with our use of new tools and the refinements we make to existing tools.

Tools that have evolved for use in particular domains can be given different assignations. For example, tools designed for domestic use are often called utensils.

Observation has confirmed that that multiple species can use tools, including monkeys, apes, several birds, sea otters, and others. Philosophers originally thought that only humans had the ability to make tools, until zoologists observed birds and monkeys making tools. Now humans' unique relationship to tools is considered to be that we are the only species that uses tools to make other tools.Fact: date=July 2007

Most anthropologists believe that the use of tools was an important step in the evolution of mankind.Sam Lilley, Men, Machines and History: The Story of Tools and Machines in Relation to Social Progress, 1948 Cobbett Press. Humans evolved an opposable thumb — useful in holding tools — and increased dramatically in intelligence, which aided in the use of tools.

Functions

• Cutting tools, such as the knife, scythe or sickle, are wedge-shaped implements that produce a shearing force along a narrow face. Ideally, the edge of the tool needs to be harder than the material being cut or else the blade will become dulled with repeated use. But even resilient tools will require periodic sharpening, which is the process of removing deformation wear from the edge. Also gouges and drill bits.
• Moving tools move huge and tiny things, e.g. concentrating force tools like the hammer moves a nail, the maul moves a stake, or a whip moves flesh on a horse. These operate by applying physical compression to a surface. In the case of the screwdriver, the force is sideways and called torque. Writing implements deliver a fluid to a surface via compression to activate the ink cartridge. Also grabbing and twisting nuts and blots with pliers, a glove, a wrench, etc...) All these tools move items by some kind of force. Also Trucks, Rockets and Planes move larger items.
• Tools which enact chemical changes, including temperature and ignition, such as lighters, blowtorches and solvent sprays.
• Guiding and measuring tools include the ruler, set square, straightedge and theodolite.
• Shaping tools, such as moulds, jigs, trowels, concrete formwork, caulk, concrete.
• Fastening tools, such as welders, rivet guns, nail guns, glue guns, glue.