That is a rather deep subject and the perhaps the best answer is to redirect you to http://en.wikipedia.org/wiki/Ren%C3%A9_Girard. But I'll try a short answer here: scapegoat is the name of a general social mechanism - according to Rene Girard it is a way to relieve the violent tention inside a human group. Originally it was with human sacrifices - but later it became ritualized - and one form of this ritualization was using goats in the Jewish religion. According to Girard Jesus was a scapegoat - but the Evangelies tell the story of the scapegoating from the point of view of it's victim - showing the whole injustice of it.
Interesting, although I must say I suspect he is projecting his own issues onto the world. Like "scapegoating" might have played a big role in his life, so he sees it everywhere (classical psychological mechanism). At least it never appeared so prominent to me, somehow I doubt that it is sufficient to build a theory of human societies.
There's more to it than personal issues - it's one of the great socio-cultural insights of our time. Girard's argument is that human societies are originally based on violence and that this violence remains in ritualized form, though it is mostly no longer physical. If you are at all aware of how human communities tend to treat non-conformists, this mechanism won't seem so rare. It's a matter of noticing the symbolic and indirect forms that it takes. For me, it's a great insight because it calls each of us to be vigilant about our own capacity for violence, which goes a lot further than what we would normally label "violence".
Edit: you made another interesting point:
Also doesn't Christianity make a big deal about Jesus sacrificing himself?
I know Girard argues that this is not the scapegoat mechanism but the ultimate exposure of the scapegoat mechanism. I remember thinking this is where he starts to get too clever in defence of his own religion. Certainly it's his cultural analyses, not his religious thought, for which Girard is widely respected. But I think also that his claims about Christianity are bound up with a vigorous defense of Western civilization that in the late 20th century was an intellectually courageous position to take.
I don't think that insight is original to Girard. Nietzsche reinterpretted morality along those lines, i.e. the slaves don't have anyone to vent their will to power on, so they vent it on themselves in the form of morality. Freud's development of the ego and id are also along these lines.
True, but see my response to grusome where I explain how I see their ideas working together. My understanding of Girard's work is based on the wikipedia article:
You are right, I haven't read Nietszche state the precise idea that religion is based on scapegoating. Nietzsche does have the more general idea that religion is based on catharsis reached through violence and oppression. However, the root of violence is not mimetic for Nietzsche, but rather a common will to power that all have, but only some can act out externally.
At the same time, I can see the will to power idea working together with Girard's mimesis idea. The slaves want to be like the masters. This is the first form of mediation, the type that is beyond the reach of the desiree. When multiple slaves want to be like the masters, then you have the second form of mediation.
At that point, Nietzsche and Girard branch apart. Nietszche thinks the slaves reach catharsis through internalizing their will to power in the form of conscience and the ascetic ideal, while Girard thinks they reach catharsis through externalizing their desire for power on a common victim.
Yes, Girard thinks it is rivalry that leads to scapegoating, but if the original desire is power, then this neatly conflates the nature of the desire and the rivalry, since rivalry and the final scapegoating can also be considered the exercise of a will to power.
