A toolbar is a user interface element consisting of a strip of icons, buttons or similar controls. Usually found at the top or the sides of an interface window, they offer a quick and convenient way to access frequently used controls without having to navigate a full menu system. A browsers toolbar is a particular type of toolbar, specific to web browsers, and stays on the screen as you surf the internet.
There are, however, many toolbars you may not want and which may be smuggled onto your system as part of the installation of an unrelated program.