Originally Posted by: RazboWhy "Perfect"? What is it about Unison, 4, 5 & Octave that is "Perfect"?[/quote]
The nomenclature relates to relative consonance &/or dissonance of the intervals in question. These two Wiki articles taken together address the topic very well. Be forewarned you are wading in fairly thick music theory, physics and acoustics:

http://en.wikipedia.org/wiki/Interval_(music)
http://en.wikipedia.org/wiki/Consonance_and_dissonance
[QUOTE=Razbo]I understand moving a half step up or down from a Perfect interval is Augmented or Diminished vs. being Major or Minor, but what's the difference?