if they are a friend, you see their timeline and they see yours. If they are only a fan, they see your timeline, but you do not see theirs.

Ahh. I've just been friending and fan-ing interchangably.

What's proper Plurk etiquette?

your choice really Dan.

I would prefer Friend rather then Fan

I still don't know the etiquette on this myself! If I've never exchanged with the person, I often "fan" 'em. Also FYI, when you ask

Yeah, yeah.

to be added as a friend, the person has option to say "add as my fan" instead.