This website will look at two aspects of music in detail – Melody and Harmony. The third aspect of music, Rhythm, is touched on briefly in parts, but does not have a section dedicated to it on this site.
Melody is concerned with the way single notes sound in succession.
Harmony is concerned with the way groups of notes sound when played together.
Melody can be thought of as the horizontal part in music, and harmony can be thought of as the vertical part.
On the guitar, we have both of these factors at our disposal: we either strum the guitar to create chords (harmony), or we pick individual notes in succession to create a melody.
The ultimate technique for any solo guitarist is to combine these two factors into one fluid style, so one can then play single-note runs and the harmonies for these runs at the same time.