It is obvious where this knot got its name from as when laid out flat it looks like a figure of eight. On its own its main use is as a stopper knot, preventing a piece of rope going all the way through a hole, but when it is doubled back on its self it makes a very strong loop that is used in climbing to secure the rope to a harness. This knot is quick and easy to tie and un tie and will only come undone if subject to continuous tugging.
To make a figure of eight all you do is make a loop (image 2), pass the working end of the rope behind the static end of the line (image 3), and then back through the loop (image 1)