Every note name in Western music, with where it sits on the keyboard, what it sounds like and which other name it can be written under.
Western music uses seven letter names, A to G, which repeat every octave. A sharp raises a note by one half step and a flat lowers it by one half step, which is how the twelve pitches of the octave are written with only seven letters.
Because sharps and flats overlap, most pitches have two names. C♯ and D♭ are the same key on a piano; which spelling is correct depends on the key you are in, not on the sound.
Western music has twelve different pitches in each octave, written with seven letter names plus sharps and flats. That produces 21 note names in common use: seven naturals, seven sharps and seven flats.
A sharp raises a note by one half step and a flat lowers it by one half step. Two different spellings can therefore point at the same pitch — C♯ and D♭ sound identical.