Does not exist. If it did exist; the definition would be 'someone trying to be a certain stereotype'. However, most people define it in their heads as 'someone who isn't whoever/whatever they are trying to be because; 1, if they have to try then they are fake, or 2, they are trying too hard and failing'. Both of these reasons are judgmental and highly unintelligent. If using the word to refer to someone else, the user is generally saying it to make themselves feel higher than the other because the user feels insecure or low. What the word, in all honesty, really is? A lie.
Insecure 1: "Look at that girl. She tries so hard to be scene. What a fake."
Insecure 2: "Seriously. She fails so bad. What a poser."
Intelligent Life-form: -laughs- "Its funny because I know that the only reason you're saying that is because you're trying to make yourself look better. It's also funny since the only reason you're doing that is because you're insecure about your own self. -laughs- ...and you say she fails."
Someone classed as a wannabe is generally considered a bad person simply because they want to be something that they aren't yet. Something most people cannot seem to understand is that everyone is a wannabe at some point in their life. You make fun of that "barbie wannabe" over there thinks she has to have big boobs or be really skinny to be around the type of people she likes, she will learn to make it her own as time goes on. You make fun of that girl covered in makeup and stuck in class faking a hangover because she thinks it will make her popular, she will learn either by friends, family, or time that it only makes her look... significantly unintelligent. I'm sure at one point in your life you've tried hard to be someone or something when you weren't quite there yet, right?
Ignorant 1: "Dude, that girl's makeup is way too dark. She thinks she's goth. She's so retarded."
Ignorant 2: "Ugh, I know, right? She's such a wannabe."
Intelligent: "You say that like you never wanted to be a super hero as a kid. Wanting to feel accepted isn't retarded. Judging someone for wanting to start defining themselves differently is. It has to be the most hypocritical thing a person can do."