In Western music, there are twelve notes, or twelve divisions in sound we hear as equal steps in pitch in an octave. They are named alphabetically in ascending pitch order as follows: A, A# (A-sharp), B, C, C#, D, D#, E, F, F#, G, G#. On a guitar, each fret represents one half-step or semitone. A whole step is equivalent to two half-steps or two semitones. Study the octave diagram above, an octave is the twelve semitone distance between repeating notes (A to A, B to B, etc.).
Though naming the notes is absolutely necessary in music, memorizing every single note on the fretboard is very difficult and does little for the understanding of theory and exploration of music, especially when alternate tunings can change the locations of notes. Study the interval diagram above. In this book, the interval diagram is used in lieu of traditional written music to show the combinations of different notes. Become very familiar with what it represents because it is used often.