When one speaks of Christianity it's important to remember that this isn't a monolithic movement. Besides there being Orthodox, Catholic, and Protestant, from there there are many further subdivisions and further subdivisions. Just think, there are dozens of varieties of Baptists, alone.
I've posted earlier about my readings of Brian McLaren, one of the central figures in the Emergent movement, but as Scot McKnight of North Park Theological Seminary, a self-described but critically engaged emerging Christian, offers an interesting synopsis of the movement on Christianity Today's blogsite.
I'll just put up the five streams that he believes flow into the Emerging lake:
3. Praxis oriented (worship, orthopraxy, missional)
4. Post-Evangelical (their not caught up in a specific theology)
5. Political (that is, they tend to vote overwhelmingly Democrat, because they feel that the Dems put more emphasis on things like poverty and the environment).
Take a look, if you're interested in understanding this interesting movement that's transforming important sectors of Evangelicalism and has been influencing Mainliners as well. In fact, in many ways it serves as a bridge between the two sectors of Protestantism.