In music, we give the notes names based on the first 7 letters of the alphabet; A, B, C, D, E, F and G. When we reach the end, we just start over again.
However, there are actually 12 notes in total, and together these form what's called the chromatic scale.
We have already covered the first 7, so what about the other 5?
The remaining 5 are the sharp # and flat @ notes, and fit in between most of the original 7.
The thing to remember with these notes is that they can be either sharp or flat.
If we start at A and move up, the chromatic scale looks like this...
A A# B C C# D D# E F F# G G# Bb Db Eb Gb Ab
There are a couple of important things to notice that will help you remember the chromatic scale in both directions.
First, in both scales, there is no sharp of flat note between B and C nor is there one between E and F. Every other note has one in between.
Second, the sharp and flat notes can be spelled two different ways, even though to our ear, they are the same note. These are called enharmonic. E.g. C# and D@ are written differently, but are actually the same pitch (i.e. they sound the same).
The chromatic scale can be played on the guitar by starting anywhere you want, and moving up one fret at a time until you have gone through all 12 notes. The distance from one fret to the next is known as a semitone.