The Prusik Hitch

Alpinists keep a variety of technical tools in their back-pockets. One of the most important tools is the prusik hitch. The prusik hitch is most commonly used for crevasse rescue systems on a glacier, for rock rescue systems, and for climbing fixed lines.

Climbers practice climbing fixed lines with prusiks.

The following video is a bit drawn out for what it is, but definitely provides a good tutorial on how to tie this incredibly important hitch.