A fanbrat is a young, exceptionally disruptive fan of any series, or someone who behaves as such. Fanbrats are usually preteens or young teenagers, although not all young fans are considered fanbrats.
The defining characteristic of a fanbrat is immaturity beyond what would be expected out of a normal fangirl or fanboy. Where a normal fangirl may behave marginally badly or be hurt if her work is criticized, a fanbrat will pull a full-on temper tantrum against the criticism in question. Sometimes it is possible to see evidence of such a quarrel in their fanfiction itself: real life fights may be depicted, and the objects of their frustrations dispatched callously.
Fanbrats are most commonly found in any fandom aimed at their demographic, such as Disney Channel shows, but can also be commonly found in anime and other fandoms. They tend to try to make up for their lack of life experience by pretending to be as worldly or as 'mature' as possible... though they do not succeed in seeming more grown up or 'cooler' than they really are at all.
Some refer to them as the fandom equivalent of 'n00bs,' tending to lack basic knowledge both of their fandom and how to get along with others in the fandom community.
A fanbrat cannot simply be defined as a 'young' or 'casual' fan. The term "fanbrat" and the bratty attitude are inseparable, and calling anyone a fanbrat is a severe insult, even if the person in question does happen to be one. Referring to a fellow Boarder or PPC Agent as a fanbrat is a grave misdemeanor. Should you wish to be eviscerated for some strange reason, that would be a fairly reliable method.
The PPC does not 'go after' fanbrats, nor does it have any official quarrel with them.