I heard that banned user are able to login but not able to send the PM and write in this forum.
It makes me wonder is there is any difference between guest user and banned user? Why forum allow login to them?
When you're banned: You can see people's trust ratings, have read only access to hidden boards(Investigations), retrieve information from PMs, and see merit histories. You can't send PMs or make posts.
IIRC, for some user, he was a VIP, and theymos had banned him, and completely blocked access of bitcointalk when he tried logging in. As in, he couldn't even see what the other banned users normally would.
Guests can only read stuff, back in the day, they were allowed to post as Anonymous. Guests can't see trust ratings/history or merit history, and can't see Investigations board.