This is an acronym for You Know You're Obsessed When...
This is usually shortened in thread
s so that a registered user of any given forum (for fans of either bands, celebrities, or the like) can talk about their list of obsessions without having to spell the whole acronym out.
Then, the registered users of said forum will spend pages upon pages describing a tattoo, sweater, hair part, etc. that whoever/whatever they're obsessed with has. And, of course, the rest of the site members will join in to say that the part is actually on the other side, and that you're a fool for not knowing and then you will immediately be shunned from that site for not being a TRUE *cough*stalker*cough* fan.