Plainchant is a form of early Christian music which is monophonic in form, with a single melody sung by an entire group of performers, classically without accompaniment. Plainchant was used to recite the Christian liturgy well through the 13th century, when polyphonic modes began to dominate. You can still hear plainchant, also known as plainsong, in some churches, and a number of musical groups have revived it, making recordings for people who appreciate this unique and very distinctive musical form.
Plainchant actually predates Christianity, and a number of religions have incorporated some form of plainsong into their liturgies and ceremonial singing. The definitive characteristic of plainchant is that it is monophonic in form, with many voices blending together as one. Some people find plainsong quite moving to listen to, especially when the performers are all talented and well trained.
Pope Gregory I is heavily associated with the history of plainchant, and he lends his name to a particular style of plainsong, Gregorian chanting. Gregory I did not actually develop this chant style, but his name was likely associated with it because of a mix up between him and a later Gregory. Even as Gregorian chanting arose, however, other styles continued to be used, although Gregorian chanting is the official chant of the Roman Catholic Church.
Plainchant compositions are typically designed for only male voices, reflecting the male monks and priests of the early Church. A number of different pitches and modes have been used historically in such compositions, with different styles of plainchant being utilized, depending on the formality of the liturgy. Classically, plainsing is performed in Latin, as this language was used extensively by the early Church to celebrate the liturgy.
While plainchant is very simple, utilizing only one melody and voices with no accompaniment, it can be very beautiful, especially when performed in a cathedral, where the acoustics are often ideal for amplifying the voices. It should come as no surprise to learn that many people in the early Church offered up their performance of plainsong to God, much as some modern churches continue to do with their singing.