The term Byzantine chant refers to any chants used in the Christian Church that relates to the Orthodox religions. These religions use the chant right after performing a rite and is considered sacred in the Church. Its name comes from the fact that Byzantium and Constantinople was the first place where the music was used. These people began using chanting in 330 and continued using it in their religious ceremonies until the area was destroyed in 1453.
Chants used today date back to at least the ninth century as there are still manuscripts from this area that exist. Church histories dating back to even earlier times prove that groups used the music in their religious ceremonies. Many of the works contained passages from ancient Greek poetry, but historians believe that other passages are basically nonsense because they don't have a possible translation. It's similar to how modern day chanting sometimes uses guttural speaking as a form of sound. Byzantine chants are made of individual stanzas wound together to form verses known as troparion.
During the medieval period, chants were based on the idea of connecting with God. Chanters believed that the words they spoke and sang during rituals were transmitted directly to God and the angels above. There are even instances of chants that contained messages relating to angels. The unique thing was that the writer of the chant was often kept hidden or secret because they believed that individuals shouldn't take pride in religious music. However this also changed music because it was the first time in history that individual hymns were set to music.
Byzantine chant uses eight different tones that sound significantly different. These tones are also known as modes and are used at different times. The first mode begins on the day after the Sunday of St. Thomas and continues to change each week. On Bright Week, the modes change each day, starting with the first mode on Sunday and ending on Saturday of that week when the eighth mode is used. The seventh mode isn't used because it sounds significantly darker than the others.
Resources on Byzantine chant include:
Byzantine chant is often accompanied by instruments and in some more popular churches they even used modern day instruments like Gibson guitars. It's interesting to note though that many of these chants date back to ancient times, but were persevered and passed down through the church. The authors of these older songs aren't remembered, but their work continues to live on.