The prusik hitch may be pretty basic for many readers, but there are certain essential knots that every climber should know.
The American Alpine Institute has posted a great video illustrating the prusik knot.
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.
In addition to their stated uses, I would add that my two most common uses have been to back up a rappel and to ascend a rope after a sketchy (and usually ill-advised) rappel.
Trust me, make sure you learn it!