There are many different skill level systems that have been created by different unicycling organizations. This page will go over all the ones you might come across and which ones are most used.
Official IUF Skill Levels - The official skill levels used by the International Unicycling Federation, as well as the Unicycling Society of America. These skill levels are geared towards freestyle, and are decent, but not as reflective of the freestyle riding of recent decades.
USA Flatland Levels - The flatland levels were conceived by Dale Gryzch and Scott Julian, and are a well thought out skill level system that represents the full spectrum of flatland skills popular in today's flatland style.
Hutch Trials Levels - The Hutch Trials levels were created by Andy Cotter for the (no longer existing) Hutchinson club from Minnesota which specialized in Trial. These levels seem to be a bit below the level of today's trials riding but are overall pretty solid. These levels also have a unique testing method available that allows testees to test a level over long periods of time, instead of all in one go.
IUF Proposed Levels/Extreme Levels - I won't even link these they are not good