Overhand | Double Overhand
This simple knot is commonly used to finish the end of a rope so it does not fray. It is also used as a stop knot to keep a rope from slipping through the end of a pulley. Because of its tendency to bind, it is typically only used in areas (like finishing off the end of a rope) where it is intended to be permanent or in situations where it will not be subjected to a great deal of tension. It should never be used in situations where it might be holding the weight of a human (such as if you are doing a suspension).
Pros: Finishing a rope this way is that the lump formed by such a knot gives greater control over the ends when tucking them under other lines at the finish of a tie.
Cons: The down side of this approach is that there is a lump at the end of your lines which puts a bit of a speed bump in the way when pulling tails through a tight area. It also has the tendancy to bind if the rope is put under strong tension and therefore be difficult to untie. An overhand reduces the amount of tension a rope can withstand (its strength) by 50%.
Basic Overhand Knot
Double Overhand Knot
Overhand Knot on a Bight