A slip knot (or running knot) is one of two different classes of knot. The most common are knots which attach a line to an object and tighten when tension is applied to the free end of the line. The other is a type of knot designed to bind one end of a rope to the middle of another, while allowing the knot to slide along the rope.
The former kind is generally created by attaching a rope to itself, creating a loop which can be tightened later. Any knot which is used to secure a line to either a post or ring (starting with the simple half hitch, and including such knots as the bowline and clove hitch) can be turned into this sort of slip knot by tying it around the standing part of the line.
The most common example of the second kind of slip knot is the rolling hitch.