a girl who hollas back at a guy who already is taken and she knows he is. so if your man asks one of your girls out and she says yes, shes a hollaback girl. because she holla'ed back when she knew he was taken.
this is a person who is at a sporting event and is in the crowd and yells back to the cheerleaders when they yell to the crowd to spell the teams name.see eg. The reference to this in the song by Gwen Steffani is that she believes cheerleaders think that they are better than her and were talking about her behind her back and she doesn't want to be thought of as lower than they are and that is why she ends up becoming a cheerleader or dreaming of becoming a cheerleader. Its pretty stupid but I think that is what it is about.