The bowline knot is a very useful and very simple knot. The bowline knot forms a loop at the end of a line. What makes the knot so very useful is that the size of the loop will not change when shocked or loaded. No slippage!!! Also the loop can be easily untied after having been put under a heavy load.
There is one drawback to the knot and that is it can work itself loose when not under a load. So a boat bouncing around in the surf would eventually become untied.