Re: diminished chords. im looking at you "DiminisheD-7th"
who? me?! 

Diminished - tr. 1. a. To make smaller or less or to cause to appear so. b. To detract from the authority, reputation, or prestige of. 2. To cause to taper. 3. Music To reduce (a perfect or minor interval) by a semitone.
Verb
intr. 1. To become smaller or less. See Synonyms at decrease. 2. To taper.
a diminished chord is bassicaly a major chord that is degraded down (if that sounds right).
if the degrees in the major would be 1 - 3 - 5, the diminished would be 1 - b3 - b5.
a dim7th chord would be the dominant 7th chord degraded
1 - 3 - 5 - b7 --->  1 - b3 - b5 -bb7
major7 ^                     dim7th
notice the bb7 on the diminished chord. tonally its the same as a 6th, but its not written that way, because its a flat - b7th.
i cant realy describe the sound of a diminished chord, its sort of evil sounding. the sound is very special, and when you know what they sound like, you'll probably spot them pretty much anywhere. listen to Malmsteen and Michael Romeo they abuse it thoroughly.
the diminished arpeggio's usually fit well on leading tone situations, where minor harmonic would also fit.
there are also augmented chord, which are "enlarged" major chords
1 - 3 - #5 - (7)