A clove hitch is a quick and simple knot. Usually used for tying into belays and is handy for when you want a knot to be adjustable.
So why a 'clovehitch'? Well an old treatise referred to this knot as a builder's knot but it could have got it's name from a chap called William Falconer who may have named it when he wrote his Universal Dictionary of the Marine in 1769. On board ships the knot was used to secure ratlines to shrouds, creating those rope ladders up which saliormen scrabbled to man the yardarms...