The Medieval Period of classical music refers to music from 475-1400. The documentation of music from this time frame is mainly passed down to us through religious sources. Because of the costs involved during this time frame, and the scarcity of resources for the most part only the church had formal musical arrangements, and so the popular music of this time is not well known.
The Medieval Musical form was simple, and the primary example of this is the Gregorian chant. Voices were in unison, and no formalized notation of rhythm existed. As this period of classical music progressed, the communication for music became more complete, and by the end of the Medieval Period there are the beginnings of harmonies and rhythm indicated in musical notation.
The instrumentation during this time was very similar to what we find in modern music, but of course more primitive in design. Cornets were made of wood or ivory, rather than metal, were straight rather than having a design of loops and curves, and had no valves. They were very short, and holes for playing different notes did not appear until the mid 15th century.
Flutes were made of wood, and were sometimes blown from the end rather than side blown. Recorders were pretty much the same as they are now. One instrument popular in medieval times but not commonly seen in today’s classical music was the pan flute. Stringed instruments such as lutes and mandolins were also common, and precursors of the organ, violin, and trombone existed as well.
During this period of time much of the works were written anonymously, so records of specific composers are hard to come by. The Catholic Church was the main source of inspiration for the Gregorian Chants, so you will want to go to religious sources for examples when building your familiarity with medieval music. However some composers did begin to emerge along with the developing music Notably Perotin and Leonin around 1175, and Adam de la Halle and Franco of Cologne in 1275. The year 1300 marked the beginning of the Ars Nova (New Art) and the emergence of four more composers: Philippe de Vitry, Guillaume de Machaut, Francesco Landini, Jacopo da Bologna. Although it is important to have an awareness of this timeframe, the bulk of your important works and composers will come from later classical music forms.