The Prusik Knot is a variation of the rolling hitch.
Some people use it for mast climbing.
Since it's not in The Ashley Book of Knots, I tied one around my pen and scanned it. This page used to contain a description written by me, but then Mark Anderson of the vessel Riparia actually did it and wrote a much better description, so with his permission I've replaced my version with his, slightly edited. I've added a few comments and clarifications of my own at the end. There was a thread on going up the mast on rec.boats.cruising in the fall of 1996 that could be referred to, but here's my technique. Remember that going up the mast is inherently dangerous.