This site covers various aspects of music theory: Keys, Number System, Rhythm tips, Scales and Chords.

You can even download the offline zipped version of the content from:
www.geocities.com/SunsetStrip/Palladium/6197/theory.zip

"Music which strictly follows music theory, can sound very stale and predictable. The other extreme is dangerous also. Deliberately violating all the "guidelines" or "patterns" of music theory (which would be difficult to do unless one first understood them) can sound chaotic. This chaos is not always "bad". Sometimes the "mood" of the music may call for some "chaos", but in most listener's opinions, this method should be used sparingly..."

Glossary of Guitar & Music Terms:
www.GuitarSite.com/glossary.htm

Guitar Tuition Resources:
www.GuitarSite.com/contents.htm#tuition

How to Play Guitar - Tutorials
www.GuitarSite.com/HowToPlay