The Divine Liturgy is the eucharistic service of the Orthodox Church. Although at various times and places, there have been several different liturgies, today only four are celebrated. By far the most common is the Divine Liturgy of St John Chrysostom, given this name because it reached more or less its final form when the saint was bishop of Constantinople. It is a somewhat abbreviated form (although the revision has historically been attributed to St John Chrysostom himself, this is currently held to be unlikely) of the second-most used service, the Divine Liturgy of St Basil the Great, which is used on Saturdays and Sundays during Lent. In my experience, the former takes a little over an hour or so, while the latter is about an hour and a half, possibly a bit more.
The third liturgy in use is the Liturgy of the Presanctified Gifts, which is attributed to St Gregory Dialogos, who was at one time the Bishop of Rome (aka, the pope; however, I avoid the title, as it is not so constricted in its usage in the East as it is in the West--the Patriarch of Alexandria, for example, is styled "Pope and Patriarch of Alexandria and all Africa," and papa has historically been used for any bishop), and it is essentially a version of Vespers expanded to include communion. This liturgy is served in the evening on weekdays during Lent, and in keeping with the spirit of the season it has a rather penitential character, rather than the joyful nature of the more common ones. It also differs from them in that the bread and wine consumed are not consecrated during the service, but are rather those left over from Sunday's liturgy. It is not now served in all parishes, and indeed it harks back to an earlier time when going to church was an everday thing, for its main purpose is so that the faithful will not be deprived of the Eucharist during the week.
The fourth is the Liturgy of St James, which, although generally regarded as the most ancient of the ones still in use, is hardly ever served, so unfortunately, I don't know much about it, aside from the fact that its Cherubic Hymn has been made into a (mostly unsung, at least in the American South) Christmas carol called "Let All Mortal Flesh Keep Silence." So I looked around, and here is what I found:
The Divine Liturgy of St James, which was until recently only celebrated on the island of Zakynthos on his feast on 23 October and in Jerusalem on the Sunday after Christmas, is today celebrated in an increasing number of Orthodox churches. It was the ancient rite of Jerusalem, as the Mystagogic Catecheses of St Cyril of Jerusalem imply. It is still, in its Syrian form, the principal liturgy of the Syrian Oriental Church, both in Syriac and, in the ancient Syrian Orthodox Church of India, in Malayalam and English.1
It is worth stating that an Orthodox liturgy, unlike a Catholic Mass, cannot be served only by clergy. The people are an integral part of the service, and it is void without them. Also, Orthodox priests--again, unlike Catholic priests--are also forbidden by canon law from serving more than one liturgy on any given altar in a single day. But by far the easiest-noticed difference from Protestant and Catholic liturgies is that the service is almost entirely sung. This can range from simple antiphons and chanting (usually Znamenny or Byzantine, or even simpler) to full-fledged multi-part choral works capable of being performed only with practice by a trained choir.