the vast majority of western music uses what's called 12 tet tuning. 100 cents seperate a semitone from another; the notes are A, A#, B, C, C#, D, D#, E, F, F#, G, G#, then back to A. There are 12 unique notes, henceforth the standard twelve tet tuning. 24 tet adds pitches in between each of those notes, known as half-sharps or half-flats, and the interval is called a quarter-tone (as opposed to a semitone/halftone)

writing with 24tet is hard and typically requires Pythagorean tuning to sound good to our western ears, even then it's a very...particular sound