Music theory is the language of music. To excel as a guitarist theory is necessary. You'll find links to various guitar theory resources here.
It's only 13 pages about how music theory relates specifically to guitar. It's the easiest free guitar theory guide I've seen yet, it simplifies relating music theory to the guitar.
You can access the guide here:
The Essential Guitar Theory Guide
Musictheory.net is a simple general music theory website. The lessons are one to two lines with diagrams(excellent for beginners). You can learn music theory step by step with back and forward buttons.
It's very simple, and fast to grasp theory this way.
It also features ear training for notes, intervals and scales.
For this excellent free resource visit: