For many years, nearly all origami was folded in a step-by-step sequence with difficulty varying according to the complexity of individual steps and (to a lesser degree) the number of steps in the sequence. This led to the establishment of a scale of origami difficulty that has been useful for a folder to judge whether a class or instruction set was appropriate for their skill level. OrigamiUSA has long used a scale like the below for rating classes taught at our various events.
In recent years, origami has exploded with new styles, new genres, and most importantly, new ways of folding. Techniques like collapses, twists, 3D corrugations, crumpling, and more don't fit neatly into the model of sequential/step-by-step folding with 6 distinct levels. Rather than attempt to fit all of those different genres into a single scale, we've added a separate scale below for tessellations, which do seem to support a multi-level scale, but with very different skills required.
Sequential Step-by-Step Folding
Simple
For students: No previous folding experience is required to take classes rated simple.
For teachers: The models should have mountain and valley folds only, be relatively forgiving of small errors, and have reference points for everything. Even the most basic of maneuvers and techniques should be explained, i.e., say 'fold bottom edge to top edge, which is a book fold,' rather than 'fold a book fold.'
Examples: Swan, magazine cover box, jumping frog, sailboat
Low Intermediate
For students: You should have some familiarity with origami terminology and materials.
For teachers: You can assume your students have folded before, but may not know terminology. The models should be primarily mountain/valley only but with the addition of inside/outside reverse folds and squashes.
Examples: Waterbomb, star basket, tulip/leaf & stem
Intermediate
For students: All of the above, plus you should know how to fold a bird base from memory, and be good at inside reverse folds, outside reverse folds, and basic sink folds.
For teachers: You can assume that your students know basic terminology, but be prepared to reinforce. For example, you might say something like: 'fold a colored preliminary base, that is, colored side up fold two diagonal folds, then white side up fold two book folds.'
Examples: Flapping bird, crane
High Intermediate
For students: All of the above, plus you should be comfortable folding double rabbit ears, crimps, double sinks, closed sinks, and stretching a base.
For teachers: Before the class starts, explain what advanced maneuvers are included in the model, such as a closed sink, and give the students the opportunity to change to another, simpler class. Assume that the students are familiar with these maneuvers, but explain them as you go. For example, don't just say 'then fold a double rabbit ear,' but rather describe the maneuver step-by-step.
Examples: Jack-in-the-Box, Montroll 3-star models
Complex
For students: All of the above, plus you should be an experienced folder, be comfortable folding from diagrams, and be able to follow intricate sequences of precise moves.
Complex models might have "collapse" moves, where many creases come together at once. They are also commonly very sensitive to folding errors, so that if you do not fold precisely early on you won't be able to complete the model.
For teachers: Before the class starts, explain what will be expected in the class, such as folding something precisely into a grid of 64th's, and give the students the opportunity to change to another, simpler class. You should not have to explain any standard maneuvers, and should be able to state an objective and let the class proceed, such as 'please fold your paper into a grid of 64th's.' But be very aware of your students' progress, because a simple error early in the folding progression may result in inability to complete the model.
Examples: As defined by the creator of the model
Super Complex
For the student: Super Complex models are the catch-all for anything harder than the preceding. A Super Complex model might contain: Multiple nested sinks, extensive precreasing followed by collapses, and collapses with ten or more creases coming together at once.
For the teacher: Most Super Complex classes are limited, either specifically by you, or by natural selection. Don't be afraid to question each student individually as to the most complex model they've ever folded, and to gently suggest that the class might be too difficult for them. In a class rated Super Complex, you should not feel obliged to help a student who falls hopelessly behind.
Examples: As defined by the creator of the model, and especially anything that takes more than 3 hours
Tessellations
Origami "tessellations" are part of a broad category of patterned folds (sometimes subdivided into tessellations, whose creases are predominantly flat-folded, and corrugations, whose creases are predominately partially-folded and sometimes curved as well). They have their own unique challenges, warranting a separate scale and list of examples. (H/T Madonna Yoder for this information.)
Simple
Patterns of overlapping pleats.
Examples: Designs by Paul Jackson and Goran Konjevod.
Low Intermediate
Folding and manipulating grids (triangle grid on square or hexagon).
Examples: Patterns that use only grid lines - herringbone corrugation, three-way intersections of pleats. See Herringbone Corrugation (traditional, also called Miura-Ori), Spread Hex by Eric Gjerde, three-way intersections of pleats
Intermediate
Folding closed and open triangle and square twists, folding patterns with twists on one side of the paper, folding thirds on the grid.
Examples: See Clover by Shuzo Fujimoto, Triangle Twist Tessellation, Square Weave Tessellation, Triangle Waterbomb Corrugation by Ron Resch
High Intermediate
Closed and open hexagon twists, closed rhombus and right triangle twists, triangle and compound wraps, precreasing precisely, folding your own grid to bring to a class.
Examples: See Celtic Circle by Robin Scholz, Bricks by Ilan Garibi, Rhombus Weave by Joel Cooper, Hydrangea and Disintegration by Shuzo Fujimoto, Chiva Rumbera by Madonna Yoder.
Complex
Folding general sizes of twists (including trapezoid twists), isoarea twists, and hybrid twists, recognizing symmetry points in patterns, repeating a sequence of up to 5 twists, basic knowledge of tilings used in twist-based tessellations, and collapsing dozens of creases at the same time.
Examples: See Just Rhombi Flagstone by Joel Cooper, Crenulation, Herringbone Weave, and Tripartite by Madonna Yoder, patterns with hybrid hexagon twists, 1-Cube Exercise by Alessandro Beber, Flower Tower by Chris Palmer, curved tessellations by Ekaterina Lukasheva.
Super Complex
Folding mixed-depth twists. You can follow tiling-based descriptions of folding (add one layer of tiles around the center, use pseudo-tiling breaks), can autonomously continue patterns once established.
Examples: See Masks by Joel Cooper, Yoder 4-5 star patterns like Medallions, Ornithogalum Dubium by Halina Rosciszewska.