The Bowline, learn to tie it, learn it's uses, watch an animation...
The Bowline is very popular amongst sailing types because of it's properties. It is quick and easy to make, and it WILL NOT SLIP if made correctly. It's name derives from the "bow line", a rope in square riggers used to hold forwards the weather leech of a square sail.
It is often used to excess by yachtsmen for just about everything from securing the boat to a pontoon to joining two ropes together (using 2 bowlines). There are better knots and hitches for certain jobs..ie. a round turn and two half hitches can be released under load and under control. However if you need a loop in the end of a rope that will not slip or tighten on itself, and will undo easilly when the time comes, the bowline is the knot to use.
Making the bowline is often described using the rabbit and the tree scenario, and this is how we will describe it here for complete beginners.
The standing part of the rope is the tree.
There is a "hole" at the bottom of the tree. This is formed by passing the rope overhand to form a loop.
A larger loop is formed, "The Rabbit", is the bitter end of the rope.
The rabbit comes out of the hole
The Rabbit runs around the back of the tree
The rabbit scuttles BACK DOWN the hole again, pull tight, all done.
Tip: If someone passes you a bowline expecting it to go around a bollard which is much bigger than the loop they have prepared....
Pass your hand THROUGH the loop up to your elbow, GRAB the standing part of the rope, PULL it through the loop of the bowline....to make a new loop as big as required. WARNING this WILL tighten on the object, so is NOT suitable for putting around a person.