A 'belieber' is a fan girl of teeny bopper sensation Justin Bieber. They are usually between the age of 10-15. A lot of them spend most of their life on Twitter where they:
-Tweet about how much they love Justin Bieber
-Shout-out other 'beliebers'
-Use a lot of tags like #ifollowback or #teamfollowback.
Other twitter users who are not beliebers (and/or above the age of 15) get rather annoyed by the insane amount of 'beliebers' on Twitter and counter act with tags like #Justinbeibersucks.
Beliebers are also well known for fainting at Justin Bieber concerts/appearances. They are also well known for covering their room and school books with pictures of Justin Bieber.
#bieberfact he said the best thing about his fans is that we all stick together as one big family. RT if you are from the belieber FAMILY