A dagger is more a weapon made for thrusting than a tool for slicing and cutting. Historically daggers were important secondary weapons in Europe during the Middle Ages and the renaissance.
Often a dagger is fairly long, and some may verge on being of sword length. Most daggers are double edged, although there are exceptions.
A modern version of the dagger is the bayonet, which becomes a spear type weapon when mounted on the barrel of a rifle.
See also: dagger (typography)