It's a lot easier to point you 'in the right direction' than try to explain it to you.
Check out the EC&M Magazine website (ecmweb.com)...they have quite a number of articles on this subject that should help clear this up for you. This site has a 'search' engine that will take you to back issues for many years...you can search by subject, article title, author, or date.
Aug. 2000 - 'Classifying & Using Class 1,2,3 Circuits' by James Stallcup is very good.
Jan., Feb., & Dec. of 2001 - Mike Holt has several articles on 'Technology Wiring' - this includes Class 1,2,3 wiring circuits.