Gregorian chant is the central tradition of Western plainchant, a form of monophonic liturgical chant in Western Christianity that accompanied the celebration of Mass and other ritual services. This vast repertory of chants is the oldest music known as it is the first repertory to have been adequately notated in the 10th century. In general, the chants were learnt by the viva voce method, that is by following the given example orally, which took many years of experience in the Schola Cantorum. Gregorian chant originated in Monastic life, in which singing the 'Divine Service' nine times a day at the proper hours was upheld according to the Rule of St. Benedict. Singing psalms made up a large part of the life in a monastic community, while a smaller group and soloists sang the chants. In its long history Gregorian Chant has been subjected to many gradual changes and some reforms.
more : http://en.wikipedia.org/wiki/Gregorian_chant
Guido of Arezzo (991/992 – 1050) came up with a method for teaching the singers to learn chants in a short time, and quickly became famous throughout north Italy. However, he attracted the hostility of the other monks at the abbey, prompting him to move to Arezzo, a town which had no abbey, but which did have a large group of cathedral singers, whose training Bishop Tedald invited him to conduct.
more :http://en.wikipedia.org/wiki/Guido_d%27Arezzo
By the 13th century, the neumes of Gregorian chant were usually written in square notation on a staff with four lines and three spaces and a clef marker. In square notation, small groups of ascending notes on a syllable are shown as stacked squares, read from bottom to top, while descending notes are written with diamonds read from left to right. In melismatic chants, in which a syllable may be sung to a large number of notes, a series of smaller such groups of neumes are written in succession, read from left to right. A special symbol called the custos, placed at the end of a system, showed which pitch came next at the start of the following system. Special neumes such as the oriscus, quilisma, and liquescent neumes, indicate particular vocal treatments for these notes. This system of square notation is standard in modern chantbooks.
more : http://en.wikipedia.org/wiki/Neume