Round Turn and Two Half Hitches
Not a very pretty name for a useful knot. It's used to tie up your boat on a ring or other fastening. I use it to quickly attach bumpers to the side of the boat. It's one of the multi purpose knots and its not too hard to take apart. Its better if there is some tension on the knot otherwise it can wiggle free. This is probably the second knot you will learn if you take a sailing course after the Bowline.
Making the Round Turn and Two Half Hitches Knot.
Adding more turns or more half hitches makes the round turn and two half hitches knot more secure. Keep making the half hitches in the same direction.
A nice thing about this knot is that you can throw a few turns on a ring and they will take the tension off and you can get the half hitches on with one hand. Another nice thing about it is that it's easy to remember how to tie this knot.