The degrees of notes

in context of C-Major key:

C: 1st degree often referred as "root".
D: 2nd degree (9th degree when played on top)
E: 3rd degree
F: 4th degree (11th degree when played on top and combined with minor 7th)
G: 5th degree
A: 6th degree
Bb: minor 7th degree often referred as flat-7th or b7th in Major key.
B: Major 7th degree
Eb: b3rd degree.

The C, D, E, F, G, A, B notes are the so called diatonic notes in C-Major key.
We can find that the songs are containing many non-diatonic notes (eg. Bb and Eb).