There are two definitions to this. The first is just a girl who is well known, the second is the stereotypical bitch we all know oh so well.
1. A well known girl. She may be talented somehow, or be really (naturally) pretty. She may be really intelligent. She may have an interesting personality or have a great fashion sense. She may just have a lot of friends. That's it. She isn't the stereotypical bitch I will talk about next.
2. A brat. She is probably popular for either having rich parents (by the way, once she gets out of high school and the parents stop paying for everything, she most likely will become broke), a popular older sibling, or sleeping around and spreading gossip. She loves spreading gossip. She most likely dresses preppy. Like Abercrombie, Aeropostale, and Hollister. Gets awful grades and is unintelligent out of school. Boring and bratty personality. She usually has a stick thin body and makes fun of curvy girls. She cakes her face with makeup. She usually acts bubbly, loud, and obnoxious in real life but tries to act deep, depressed, and nerdy (while mocking depressed people, nerds, and intelligent people) online. She looks down upon anyone or anything that isn't popular. She only listens to the Top 40 pop. The only books she reads are the Twilight Saga. Same with movies. She sometimes may pretend to be a "gamer girl" just to get guys, but she doesn't play games. Usually a Christian (I'm not saying all Christian girls act like this of course).
I hate examples, so I won't do one. Popular Girls.