Originally posted by Monk
Extreme selfishness, with a grandiose view of one's own talents and a craving for admiration.
Also known as an attention whore.
Narcissism is more vanity though, not really anything to do with the attention of others. A narcissist will believe he or she is the best thing since sliced bread whether or not you agree or care to agree. They are content in their own mind to be the best.
A megalomaniac is someone who has delusional fantasies about their own power and an egotist is someone who drives to enhance their own self-importance. An egotist would try to be the center of attention, whereas a megalomaniac would believe they are the center of attention. Hence why a narcissist would be a megalomaniac egotist - they would believe they are the center of attention whether or not you care to agree and will be so full of themselves that dispute will be pretty much useless.
To put it in a bit less of a psychological perspective:
A narcissistic girl would think she is the best thing on the planet.
An attention whoring girl would wear short shorts and spout self-pity to fish for a compliment and attention from others.
Hope this helped.