with friends you see each others plurks, they see yours, but you don't see theirs on the timeline.

what is so weird is that I just asked the same question...and happened upon this answer. altho I can't say I understand it...

a friend is a mutual relationship, prob someone you know, a fan is someone interested in you. Also you can plurk just to your friends.

so 1) if you're my fan, you see my plurks, but I don't see yours, and 2) if I want to, I can plurk something exclusively for my friends?

yes, to change who the plurk goes to click "plurk privacy and options" under the input box