The prusik knot is used most often by climbers for self rescue situations. This knot enables a climber to ascend a single rope. This would be useful, for example, for extricating oneself out of a crevice in a glaciated region. A prusik knot would also come into play if a climber fell on an overhanging section of a rock wall and swung out of contact with the terrain. Being able to climb back up the rope to the anchor would be very useful indeed. Mechanical ascenders can also be used for these types of self rescues, but a lightweight loop of rope is many times more practical than carrying the extra weight of ascender (not to mention the expense). Therefore, I believe it is very important for any climber who ventures into the wild to have a knowledge of how to tie a prusik knot. You never know when your life could depend on it. Below is a five part illustratioin of the "Prusik Knot."
First create a loop using a lightweight determined length of rope. You need to make use of the "Fisherman Knot" as shown above to tie the ends together. Then begin to wind the loop around the main rope (in red) shown to the left.
Next, continue to wrap the loop of rope around the main rope. The more the wraps, the more the friction. Two or three wraps should be enough.
Above shows the prusik knot nearly complete. From here all you need to do is pull the coil from the right, and tighten it up as shown in figure #4.
Prusik knot completed.
The image to the left shows the prusik knot in action. A carabiner can be attached to the prusik loop along with a sling, and the climber can then place a foot in the sling. Two of these configurations must be used, one for each foot, in order to ascend the rope.