Alas, I have reached a stage in my life where keeping up with what’s happening in the DC universe is no longer possible (generally, I don’t have the opportunity to read comics at the magizine shop anymore). The last era I’d be able to explain in any detail was roughly the time Kyle Rayner became Ion; the whole Infinite Crisis deal I have only heard pieces of. Given how things get multi-retconned I’d like to at least have a general idea of what’s what and who’s who so I don’t one day discover that some major change has lasted long enough to be canon.
52 had a “history of the DCU” backup that was current up to the most recent Crisis. It focused most on the recent crises rather than general history, but it’s probably your best bet for an official source all in one place.
Otherwise, Wikipedia’s pretty good for these kind of things.
I understand there was a hush-hush project dedicated to producing an n-dimensional matrix designed to map DC continuity. Unfortunately, the matrix collapsed in upon itself and formed a quantum plothole that began inexplicably emitting showtunes and puffs of chartreuse smoke.
I hear they’re blaming it on Superboy, for some reason.