Much like we measure geographical distances in Kilometers, we measure musical distances in ‘tones‘ and ‘semi tones‘. London to Paris is 350 kilometers, C to D is one tone. They are distances between two notes.
Here’s how it all works, here we have a list of all 12 notes in a row:
A A# B C C# D D# E F F# G G# A
NB. B and E do not have sharps, this is easy to remember as they are the top two strings on the guitar.
A semi tone is a next-door-neighbour, a tone is next-door-but-one. For example:
A – A# = a semi tone
C# – D = a semi tone
E – F = a semi tone
A – B = a tone
C# – D# = a tone
E – F# = a tone