Each note is given a name. A letter from A
to G. Notes one octave apart have the same letter for a name but
sound higher or lower depending on their frequency.
You will see from the example shown below,
that as you move from left to right across the keyboard each note
of C has twice the frequency of the previous (lower) note of C.
This is the same for D to D, or E to E etc.
Octaves are divided up into a 'chromatic'
scale of twelve frequencies (or intervals) called semitones (thus
notes can be said to be one semitone apart in pitch). An interval
of two semitones is called a tone.
On a piano keyboard the 'white' keys play
notes that are one tone apart, except between B and C, and E and
F which are only a semitone apart. The 'black' keys play notes
that are a semitone apart from the note played by the two white
keys either side of them.
When played together some notes have
frequencies that interact well with each other making a 'pleasant'
sound, these can be used as chords. Other combinations of notes
have frequencies that don't interact well with each other making
musically unpleasant or discordant sounds.