From DT Online
They are used to create surface decoration by incising lines into metals by silversmiths and gunsmiths for example. They are also used by printmakers to cut copper printing plates or wood printing blocks. Jewellers use them not only for engraving but also to set stones - see The Ganoksin Project.
In use, it is typically held at approximately a 30° angle to the surface. The index and middle finger typically guide the shaft, while the handle is cradled in the palm.
Safety Point! Always make sure you are not pushing sharp tools towards your hand!