The Slip Knot
A slip knot, also called a running knot, is a broad classification of knots that basically refers to any knot that can "slip" along the rope, and/or can be untied by simply pulling one of the ends. This particular slip knot is sometimes referred to as a simple noose, and has wide range of applications. If you do a little forensic knot analysis on this knot, you will see that it is very, very similar to the overhand knot.
Knot Video: How to Tie the Slip Knot
Knot Tying Instructions
Step 1 - Hold a length of rope out in front of you and twist to form a loop.
Step 2 - Reach through the loop, grasp the dangling right end by the middle, and pull it through the loop, being careful not to let the end come through. Keep your left hand steady, and a knot will tight arond the part you are pulling through.
Step 3 - You can now hook the new loop around things (including useful things like your thumb!), and pull the main length of the rope to tighten around them.