During ischemia, the stage is set for a disaster during reperfusion. ATP levels return to normal fairly quickly, but the return of oxygen to the picture results in a burst of free radicals that burn up membranes and activate all sorts of horrible processes. One handy--if somewhat artificial--way of looking at this biochemical train wreck is to group all these damaging processes into four main categories--which I like to call The Four Horsemen.
(To meet the Four Horsemen and the Angel who's gonna punk all of 'em, just click on their heads.)
This is the part where some people, most of whom are smarter than me anyway, get all huffy and say that excitotoxicity should be a Horseman, too. They have a point, but I would argue that there's little evidence that the glutamate surge does any real damage in and of itself. Rather, excitotoxicty triggers other processes that ultimately kill the cell (the Horsemen). Besides, Durer only drew Four Horsemen, and I already filled all the saddles. It's my analogy. I can do what I want with it.