An obsessed fan who will not admit they are obsessed with a certain celebrity, but knows it deep down inside. These Justafans know more about the said celebrity themselves do, and annoys the hell out of everyone around them by talking about them all day long.
Justafan: Man I LOVE Tom Kaulitz from Tokio Hotel!! He is 21 years old, has over 350 caps, has a dog named Scotti and a cat named Kasimir, his twin brother's name is Bill who also plays in the band... *30 minutes later* ..and have you ever noticed his sexy scar on his right cheek??
Person: Goddammit I don't care!! You're seriously obsessed with that Tom person.
Justafan: No, I'm not obsessed. I'm just a fan.
Google "Tom Kaulitz facts" and click on the second page that appears. A very good example of justafan can be found there.