A tool or device is a piece of equipment that (most commonly) provides a mechanical advantage in accomplishing a physical task. The most basic tools are simple machines.
Philosophers once thought that only humans used tools, and often defined humans as tool-using animals. But observation has confirmed that monkeys, apes and other animals, mostly primates, but also some birds (ravens, for instance), and sea otters can use tools as well. Later, philosophers thought that only humans had the ability to make tools, until zoologists observed birds and monkeys making tools.
Non-physical entities such as process (improvement or reengineering), information architecture, creativity and learning itself are all invaluable tools that we humans use to better ourselves individually and collectively (or collaboratively). Certainly, the term "tool" should not be limited strictly to physical objects, but also cognitive methodologies.
Most anthropologists believe that the use of tools was an important step in the evolution of mankind. Humans evolved an opposable thumb (useful to hold the tools) and an increase in intelligence (aiding in the use of tools).
Tools can also be purely cognitive, such as a written language.
Often by design or coincidence a tool may share attributes with one or more other tools in terms of their basic functionality. In this case, some tools can substitute for other tools, either as a make-shift solution or as a matter of practical efficiency.
In many cases, the designed secondary functions of tools are not widely known. For example, many wood-cutting hand saws integrate a carpenter's square by incorporating a specially shaped handle.
Mechanical devices, though known to Alexandrian Greeks, experienced a major expansion in their use in the Middle Ages with the systematic employment of new energy sources.