A Prusik is a friction knot used primarily by climbers or people working off the ground. But they can also be used for raising and lowering equipment. Prusiks can easily and quickly be attached and detached from a rope.
Prusik's will work well on all diameters of rope that they tie around. Typically the prusik loop is of the same size or smaller than the rope or ropes it is tied around. Prusiks are designed to move freely up and down a rope until a load is placed on them, such as a climber loosening his grip and falling.
Caution: Prusiks are prone to slipping when used on frozen wet ropes.
After being put under a great deal of weight, the Prusik can be somewhat difficult to loosen again.
The Prusik is tied by wrapping the loop of rope around the vertical climbing rope a number of times, and then back through itself, forming a barrel around the rope with a tail hanging out the middle.
Pictoral How To Tie A Prusik