Tool (n) - 1. A mechanical device intended to make a task easier, 2. equipment used in a profession, e.g., tools of the trade, 3. Something to perform an operation; an instrument; a means
There are several large branches of tools that a DIY-er might be concerned with, but they can be easily split into two categories: Power tools and Hand Tools.
Power tools use electricity, gasoline or some other form of chemical energy, or compressed air to perform a task. They are usually but not always man-portable. They may also use some form of mechanical energy provided by human muscles to complete a task. Examples would include a power drill, a nail gun, an orbital sander, or a gas-powered auger.
Hand tools are man-portable tools that are operated using solely physics and mechanical force provided by human muscles. Examples might include a hammer, saw, or screwdriver.