Origami has a number of standard difficulty ratings that are loosely defined. This page on the topic by OrigamiUSA provides a good understanding. Number of individual creases (or preferably crease steps, a crease or series of creases that can be made by one straight or curved folding action) or diagram steps can be used to approximate difficulty.
Simple are the easiest and should require no previous origami experience. The most difficult maneuver is typically some form of reverse fold or basic squash. Well-known simple models include many traditional models such as the fortune teller and paper airplane.
Intermediate models require some previous origami experience and general greater dexterity and comprehension. It is divided into low, mid, and high and thus constitutes the broadest classification. Most designs you will encounter will be some level of this class.
High intermediate designs include John Montroll's one-piece omega star and Stephen Weiss's Girl in a Dress.
Complex models are typically sensitive to small inaccuracies and often contain complicated collapses. They include Jeremy Shafer's Folding the Square Base.
Supercomplex is generally defined by OrigamiUSA as "anything that takes more than three hours." Well-known supercomplex models include Robert Lang's Black Forest Cuckoo Clock and Satoshi Kamiya's Ancient Dragon.