On the question of the distinction between diminished chords and diminished 7th chords, check HERE and HERE. And there is a wealth of good information HERE. And some more HERE. If you search for "music theory" in google.com, you'll be inundated with good stuff.

The sites I've linked to are just a few of the ones available. They can provide the answers to most questions about music theory (scales, modes, intervals, chords, and the terminology that trained musicians use). They are specifically for guitarists, but the information is consistent with formal music theory. I recommend highly.

Don Firth