Check out Scot McKnight's four part series on defining the Emerging Church. I think it's one of the best explanations of the emerging church around. I've given a blurb from each post, but they are all excellent and should be read in full.
One of the reasons so many are frustrated with the Emerging Movement’s definition is found here: it is a movement concerned with praxis and not simply theology. If the older fashion was to define others by their theology, the Emerging Movement wants to be defined by its behavior. This is a dramatic challenge to the Church.
Whatever the Emerging Movement is, it is clearly a protest movement. Sometimes it can appear to be cranky, but there is substance and there is focus in what the Emerging Movement is protesting. And, though sometimes the resolutions fall flat or fail to materialize or collapse into the unworkable, there are genuine resolutions being worked out.
Tenth, the Emerging Movement wants to be Worldly. Not in the Johannine sense or in the Pauline sense, but in the Kingdom sense: it knows that God is working to restore the entire creation into an expression of his glory and so it summons everyone to participate in the grant work of God to restore and redeem. It embraces culture and state and politics and business and it protests old-fashioned Christian separationism and enclave Christian circles. The walls between Church and World, so it is suggesting, need to be impermeable and not permeable, they need to be knocked down so the passage from one to the other is an imperceptible as the passing of Jesus from one person to another.
There is nothing that should be more welcome to orthodox Christian theology than the contention that meta-narratives cannot be established on the basis of some kind of universal reason independent of faith. This is somewhat Augustinian: I believe in order to understand. It is crucial to the way of Jesus that we must first trust him in order to know him and to know ourselves and to know our vocation in this world.
I will say this again: the Emerging Movement is not entirely postmodernist in its epistemology, and it is sloppy and unfair to say that it is. What the Emerging Movement, and almost universally, is an attempt to “do church locally” in light of the postmodern condition of our world.
It can be said that the EM is theologically driven by a reaction to the sort of theology that flowed from the ancient creeds into the Reformation and from the Reformation into the present Evangelical culture. And that theology is often abstract, systematic, and rooted in logic and reason. The EM wants to root its theology, which is more practical than it is theoretical, in the incarnate life of Jesus himself. It wants a theology that is shaped by personhood and relationship rather than just rationality and systemic thinking.