Bowline Knot
The bowline knot is a simple knot for making a reliable loop. 
Bowline knot
Resuce knot - named because this knot is useful in rescue situations
Use the bowline anytime you wish to have a loop that will not slip.  This knot is commonly used in rescue situations.
How to Tie
There are several ways to tie this knot.  One method is one-handed, but it is difficult to explain and even with a picture it is not clear.  

Stephen C. Dibler, Last update: 12/19/2002