Very easy projects require a minimum quantity of folds (fewer than 15), and feature no tricky folding. These are ideal for the absolute beginner.
Easy projects require more folds than the very easy category (10-20ish) and thus more time to complete. They may also require a few slightly tricky folding patterns.
Intermediate projects require many folds (15-25ish), and often incorporate tricky folding patterns.
Hard refers to folds that require a large quantity of folds (more than 25), and consequently a fair amount of time to complete. They may also make use of pre-existing bases (i.e. the base folds are not included - you have to know how to do that already before starting these folds) to avoid cluttering the instructions. Additionally, these projects may omit the grey crease lines, assuming the user to be familiar enough with the basic folds to not need those guidelines. Additionally, the instructions are often terse. In other words, the difficulty rating of hard assumes the folder to have a decent working knowledge of origami (practical experience with various types of folds) prior to beginning that particular project. Hard projects may also require the folder to extrapolate instructions given for one part of the paper to other parts (i.e. make certain folds, then rotate the paper 180 degrees and make the same fold but this time without the numbers and crease lines). In other words, it requires a user to "fill in the gaps" on his or her own instead of explicitly illustrating every single fold and crease like the very easy and easy projects did.