I was going through "The Musical Alphabet" and can follow the tab to play the notes, but it doesn't mean anything to me.  I don't understand how memorizing where D is, allows you to know where all of the othe notes are on the fretboard without using rote memory for all of them?

Also,  in the example given for the tutorial (attached), the G note is found on more than one string, at more than one fret position?  On the top, the "G" is on the 8th fret of the B string, on the bottom it on the 5th fret of the D string.  Can you explain and clrify this for me...sorry, but I'm a newbie and yes, you'll have to take me by the hand and walk me thorough this.