A Diva in my mind is a very influential woman in music or on the celluloids. She doesn't necessarily have to be overly demanding as some have liberated the term to mean. In my mind, Nora of Sunset Boulevard is the definition of an aged diva, another one would be Diana Ross.
And a Drama Queen is just someone who thinks they're all that, kind of like J. Lo.. but she really isn't.
Mariah Carey is walking the fine line between the two.. some days, she does some stupid stuff that just makes me want to smack her, but she's such fixture musically that you can't help but acknowledge her diva status.