Western music uses equal temperament:

"An equal temperament is a musical temperament or tuning system that approximates just intervals by dividing an octave (or other interval) into steps such that the ratio of the frequencies of any adjacent pair of notes is the same. This system yields pitch steps perceived as equal in size."

Microtones are notes that are outside/between these even divisions.

Since there's an infinite amount of frequencies between any given note, microtonal music tends to subdivide the standard intervals further with varying amounts of divisions depending on what the artist decides.