An updated overview of perf events might be nice?

I know perf events / performance counters (as they previously were called)
have come up before but perhaps it would be useful to have a "state of perf
events" article at some point?

When they appeared to be a counter mechanism I could understand what they
did; I know they've been renamed to perf events because they've become more
general. But now it seems they're not even performance-related - they're
starting to sound more like a "generic callback framework" that happens to
get used in some perf monitoring code.

It'd be really nice to have a from-the-top explanation of what they have
actually become in recent kernels, since I'm having some trouble keeping up!