I can't find out the difference, though I could quite guess :
•Trusted by NexonM (these members joined in 2016 - or before, when this Forum was established)
•Approved user (these members joined after 2016)
•Junior Member (new users that were barely approved to the community)
The lowest class might be Waiting to be Approved/Waiting for Approval.
Update : if they joined before 2016 and it says Approved User that means when you joined, your first post was approved (your post had to be approved first) by Forum Moderators/Admins before.