One of the more common knots in climbing is the figure eight on a bight (being tied on a bight means that it is tied without using the ends.
How exactly would one tie this knot?
First you take the bight and bring the end down.
Then you take the end and cross it over.
Then bring it back around underneath the main strand.
Finally take the end through the top loop.
and there you have it. Before using you would want to dress it and tighten it down.
If you want to see a video of the process for tying this knot, see the one here.