The blood knot is probably the most famous fishing knot and is also called the clinch knot in various parts of the world. It is probably one of the first knots most anglers learn and some people may even remember their grandpa teaching it to them like I do.
The blood knot is popular as it is easy to tie, can be used in many different types of line and is sufficiently strong for most fishing situations. It can also be tied in the dark making it quite popular amongst night anglers.
This knot also comes in a couple of improved forms, which I have posted instructions to before, including the:
How to tie a blood knot
This is a great little animated video showing the process of correctly tying a blood knot. Remember to lubricate the line before tightening.