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.

Tools are the most important items that the ancient humans used to climb to the top of the food chain; by inventing tools, they were able to accomplish tasks that their bodies could not, such as using a spear or bow and arrow to kill prey, since their teeth were not sharp enough to pierce many animals' skins.

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.

Most anthropologists believe that the use of tools was an important step in the evolution of mankind. Humans evolved an opposable thumb — useful in holding tools — and increased dramatically in intelligence, which aided in the use of tools.