This article deals with the notes on the guitar and not why you should learn them. Why learn notes on the guitar?
Sharp notes on the ‘A’ string.
Flats on the ‘A’ string.
A# = Bb
C# = Db
D# = Eb
F# = Gb
G# = Ab
By playing barre chords, powerchords and scale patterns using these notes as your starting point gives your the ‘root’ of whatever you are trying to play.
The following barre chord played on the 5th fret of the ‘A’ string would give you a ‘D Major’ chord.
If you replace the above shape with the minor version shown below(also played at the 5th fret),
you will have a ‘D Minor’ chord.
If you’d like to read more: