A hacksaw is a hand held frame with a blade held tightly at the bottom.
This example is the Stanley 15-565 adjustable hacksaw. It can hold 10″ or 12″ blades and is used typically to cut metal or plastic pipe. The blades are made of a brittle metal and care must be taken to not apply too much pressure or the blade will break. Tension on the blade is controlled by tightening or loosening the wing nut to the right of the blade in the above picture. Too much tension and the blade can snap. Too little tension and the blade will come loose.
The hacksaw has been replaced by many power tools such as the reciprocating saw but with some projects, nothing beats a hacksaw.