# indicates a sharp, which raises the pitch of a note by one fret.
b indicates a flat, which lowers the pitch of a note by one fret.
@ indicates a natural, which is used to cancel the effect of a sharp or flat.
Thus it is possible for the same notes to have two different names. (e.g., F# has the same position on the fretboard as Gb). Here is a representation of the notes on the fretboard.
A, B, C, D, E, F, G, are all "natural notes"