Beginner
Fresh start on badminton, learning how to play badminton (Grip, footwork, hit, reflex initiative, etc). This guy can't do things right yet. Especially when they're fighting with Intermediate or higher level. They don't even stand a chance.

Intermediate
Learned how to do things right. Learning more technique, tactics, better and faster footwork, using wrist instead of shoulder, and more experience compared to the beginners. This one is capable in coordinating themselves, and playing good. Capable in fighting advanced, at least giving a fight.

Advanced
In this level, I'd say there are certain degree of level. As I believe, those who are in Advanced is capable in putting the ball (Shuttlecock) where they want them. These people is capable in using all the basic available in badminton as part of their technique. Has a lot better footwork, speed, reflexes, tactics, etc. Basically they're on the place where they can enjoy playing badminton compared to Beginner-Intermediate.

Professional
Chosen Advanced. Advanced which is spotted by Badminton Scout, have professional Contract, playing for certain club, school, any institute, or countries. This is actually can be called another advanced, and can be defeated by advanced (since I'd say people who's actually good, and not called for International Duty, refusing to join another club, and enjoying leisure playing can still be called Advanced, which is in fact s|he's probably very good).