A Cheerleader is most commonly thought of as a brainless blonde bimbo who flirts with everyone etc.
And that may be true of some people, but it is sterotyping.
Some people think that cheerleaders must be white but this is also not true.
Cheerleading to many people is a sport, just as gymnastics and other aesthetically-pleasing sports are (e.g. ballet, dance).
Other people may also think that cheerleading is limited to America. It is not. Most schools in the UK for example do not have a Cheerleading Squad, but there will be opportunities to do this as a Sport in the Local Leisure Centre.
Other things to note: Cheerleaders aren't always blonde. That is the sterotype. They are not always preppy though I doubt that anywhere would have a punk Cheerleading Squad...
Cheerleading is often used in Media. It has featured in:
The Princess Diaries Movie (in a pretty negative form)
Bring It On (in a pretty positive form)
The Princess Diaries Book (in a pretty negative form to begin with, then more neutral)
The video for Nirvana - Smells Like Teen Spirit.
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Other reasons for negative impacts on Cheerleading: They can make enemies easily. You cannot live without someone hating you, for some reason. These cheerleade...

A slutty, snobby girl who is usually self-centered and typically a flunker at school. She makes a great fuck and can even give you free head if you are a football player.

A girl in who joins a cheer squad for school events for the purpose of raising school spririt. But fails. The individual cheerleader tends to be snobby, dyes her hair blonde for no apparent reason, drive needlessly expensive cars(and usually crash them a year later), strictly hang out with the sport jocks and other cheerleaders, and stereotypically a whore who wears almost no clothes outside of school in order to attract Saturday night entertainment. . .

1.) Female member of a sporting team's cheer squad. Characterised as attractive, unintelligent/naive, and bitchy; highly concerned with body image, appearance, and popularity. Hyperactively supports, barracks and dances for their team during a sporting event. Usually date and sleep with jocks.

2.) Female equivalent of fanboy. Derived from the cheerleaders' blind faith in, and support of, their team. Used in application to figures such as particular bands, actors, ideals, styles, pop culture/television shows, countries, and so on. Like fanboys, cheerleaders are unable to discuss their idol objectively and maturely, rendering conversation with them useless and frustrating. This also makes their opinion on the topic superfluous.

see also: (1) jock, ditz, (2) fanboy

One often female and found at sport tournaments shrieking and bouncing and waving retarded poofy-things. When not at games, the cheerleader can be found in groups in the back of a car making love with a jock. Cheerleaders are often screaming at the top of their lungs in a way to express excitement.

Hot chicks.

Supposedly, a delicious high school girl who jumps around in a short skirt in order to get people to cheer. Her purpose is to rally up the team to win.

Of course, this is the "suppose" version. In reality, she is a hideous bitch that has a two digit IQ. She is most likely overweight and she couldn't get the team to win even if she offers her soul to the devil.

Why can't we go back to the good ol' days of cheerleading? Ugly girls would be excluded from cheerleading and their bubbly idiocy might actually extract enough pity from people to get them to cheer with something resembling real enthusiasm. Of course, they'll still bitches and will probably marry into wealth greater than anything the guys with IQs that exceeds their social security numbers will be able to acquired, but hey, at least they'll be cute.