Gregorian Chant has no time signature, and is sung freely. They are usually sung at masses and requiems, but are frequently used in choral music. If sung by more than one, Gregorian Chant usually doesn't have harmony, unless it's just an octave difference. Most people are familiar to ones sung by males, but both sexes sing in Gregorian Chant.
It is also used a lot in processionals, again, at masses and requiems. The free beat makes it easy for one to walk without having to look like the Marines.
The liturgical tradition which the Church has bestowed on us is a vocal, monophonic music composed along with Latin words coming from sacred texts. This is why Gregorian Chant has often been called a "sung Bible".
What we call Gregorian chant today first appeared in the Roman repertory of the fifth and sixth centuries. Its care and perhaps some of its composition was in the hands of a group of ministers in a specially dedicated service to the Roman basilicas, the schola cantorum. Gregorian chant also appears to have been an aural music, that is, transmitted by ear and committed to memory - like other music of the world at the time.
There are different forms of Gregorian Chant, used for different occasions, etc:
The Introit is used to accompany the priest or minister as he enters the pulpit.
The Alleluia ("Praise the Lord") is sung at Mass, it was originally a chant reserved for Easter Day. From there its use was extended to Lent, then to Sundays of the year, and weekly celebrations of the Resurrection.
The Offertory is sung during the offering.
The purpose of this chant is to accompany the procession of those distributing communion.
The Kyrie is a Greek formula by which the faithful "acclaim their Lord and implore his mercy." Today this chant is placed at the beginning of the Mass, as part of the penitential rite, preparing the faithful for the celebration.
The Gloria is a hymn of Eastern origin may date from as early as the second century. Usually used in Midnight Masses.
The Sanctus is used at the beginning of the Eucharistic Prayer, introduced by the preface. The Sanctus is the "hymn of the Seraphim", heard in the Temple of Jerusalem by the prophet Isaiah. It invites the Church on earth to join in the liturgy of heaven.
The Agnus Dei
This is the chant which accompanies the breaking of the bread which has just been consecrated, a necessary breaking which precedes its distribution at the communion of the faithful.