The document provides an overview of medieval era music from 900 to 1500 AD, noting that the early period consisted mainly of monophonic chants and songs while the later period saw the rise of polyphonic masses, motets, and secular songs. It describes the different categories of Gregorian chant that developed based on historical time, geography, and usage, and provides examples of some of the most well known and influential chants from the era along with their characteristics.