A prep refers to, but is not limited to, anyone who dislikes or "flames" My Immortal. Or is not a goth.
At the beginning of every new chapter of My Immortal, Tara pleads with this group of people to "fuk off PREPZ ok!", and "PREPZ STOP FLAMIGNG!". Apparently, this group is somehow affiliated with the real-life prep cliques in high school. Anyone who is not overly goffic/suicidal/stanist can be considered a "fuking prep".
Tara often tries to weed out the good readers from the bad by promoting her quiz, which was created solely for the purpose of determining whether the taker is either a "goff" or a "fuking prep". Thereby asserting that anyone who is not "goffik", is therefore a prep. Preps usually wear pink and are commonly flipped off by any and all non-preps (the goffs) in their presence.
Being called a prep is presented as a terrible thing, because preps show a lack of taste and appreciation for "modern literature" like My Immortal. Even though the term "prep" is ubiquitous in every chapter Tara writes, there is only one true prep in the entirety writing of the story.
Despite Tara's threats to injure herself if "prepz'" flaming continues, the "cruel and heartless" individuals known as preps never cease to torment her.
A prep is basically anyone who isn't like Tara. These include cheerleaders and Christians, or stuck up person. Some people define the term as 'comformist' and 'image-conscious'.
Besides goths, Punks are also stated to be enemies of the preps. How to tell them apart, or if there are punk posers, is not made clear.
In My Immortal
Within the fic itself, prep is a name used to distinguish the evil non-goffik villains from the good sympathetic goffs. Even when a character is described as dark and gloomy in the books and thus close to being goffic, like Snape and Voldemort, their My Immortal counterparts are still called preps, due to being enemies to Ebony. Tara even goes as far as to describe that Voldemort is wearing a black robe, but that it was sure he wasn't goffik. Most, if not all, preps are associated with Griffindoor, though Luna Lovegood may be a Ravenclaw (if it exists in this universe).