Double Overhand Knot
Double Overhand Knot is basically an Overhand Knot with one more pass of the rope's end through the loop. Hence, it is larger than Overhand Knot and harder to untie. It's use is the same - mostly as stopper knot and sometime to prevent rope from unraveling.
How to Tie Double Overhand Knot
On the photos below, you can see how this knot is tied. Also, you can click on small photos to see larger gallery.
Procedure is similar to tying Overhand Knot - form a loop on the rope's end and then pass the end through the loop, twice.
Pull the rope to tighten the knot and Double Overhand Knot is tied.