Comments in What is a shooting board? have taken me further down the rabbit hole.
What is a bench hook and how is it used?
A bench hook is quite simply the most useful simple woodworking jig/appliance that the handtool user can make. As Chris Schwarz says about them, "After working with one for a couple weeks, you will wonder how you ever got by without it."
In addition to its main purpose as a sawing aid a bench hook can function as a basic shooting board, the large surface can be used for chopping with a chisel (why they're sometimes also called chopping hooks) and the bar can also be used as a backstop for planing, so in essence larger bench hooks can convert any suitable flat surface into a temporary woodworking area, with sawing, planing and chopping functionality. Which is a neat trick for just three pieces of wood glued together.
There are many minor variations in design for bench hooks as you'd expect for an accessory like this which is so commonly handmade by the woodworker. The one in the photo in the response by WhatEvil is actually more suited to a left-hander, a right-hander's would more commonly have the stop offset to the left like this:
Some have a stop that spans the whole width, which can be low-profile to allow planing of thinner stock:
And with a minor modification a bench hook can function as a mitre block as well:
This is a bench hook!
A very simple and handy device, it gives you a nice solid fence which you can push a piece of wood (or metal, or whatever) against, holding it steady and allowing you to saw it easily.