This is the Note Circle. It shows all the 12 notes that exist in Western music.
Notice that A# and Bb are the same note (called enharmonic if you want to be clever). They sound the same but have different names. Also notice that there is no note in between E and F or B and C. This is where the white notes are right next to each other on the piano keyboard.
Moving clockwise around the circle makes the notes higher in pitch.
The interval of a semitone (shown on the left) is the smallest step on the guitar, and is one fret. Two semitones make a tone (shown on the right), and these two intervals make up scales.
The note circle can also be used to work out any note on the fingerboard by knowing only the note names of the open strings. Simply count clockwise around the circle, moving up one step for each fret, starting from the note of the open string.
It is theoretically possible to have a B#, it would have the same pitch as C, and would be played the same way on the instrument.
This table should be memorised as it is the foundation of all music.