There's never been a universally-accepted definition in this area, but I'll tell you what they mean to me.
Alt-beginnings are rewrites or sidesteps of the Pilot; it's still "our" Lois and Clark, up until the point of the alt-beginning. Caroline's Stardust, for instance, has a Clark and a Lois who are pretty much identical to those on the show, up until a chance encounter --their first meeting -- throws a severe spin on subsequent interactions.
Elseworlds are different; they're different universes, with fresh new characters -- they're basically Clark and/or Lois, of course, but with new circumstances/backgrounds/experiences. In Janet's story The Girl Next Door, Clark is (mostly) the guy we all know -- but Lois grew up with superpowers. That's a pretty fundamental change. Or maybe baby Kal-El went to New Krypton instead of Earth, or Lois grew up in Smallville.
Alt-universe stories involve the specific Alt-Clark that we saw in Tempus, Anyone and Lois and Clarks. Usually the stories pick up post-Lois and Clarks, so they don't change his circumstances, just introduce scenarios wherein he goes on to meet/find alt-Lois. Alt-Clark was such an appealing and heart-breaking character that we want to give him a happy ending. Connections, for instance, is an alt-universe.
Distinctions can be murky, of course, and it's possible to combine them; I wrote In Any Universe, which was an alt-beginning alt-universe
-- I took the basic facts of the alt-universe but went back to rewrite it so that Lois never went to the Congo.
I think the key is the initial state of the characters. As of the very beginning of the story:
--Did the events of the Pilot happen? Then it's the canon universe, no matter how different it gets after that.
--Are L&C identical to pre-Pilot canon L&C, but the Pilot was altered or never happened? That's an alt-beginning.
--Are L&C different than pre-Pilot canon L&C? That's an elseworld.
--Are L&C identical to canon (post-Lois and Clarks ) alt-Clark (lonely, no privacy) and alt-Lois (disappeared in the Congo)? That's a proper alt-universe.
Anyway, that's the way I mentally classify them.
We've had many long argu- I mean, discussions over the issue for at least a decade now, so this is by no means authoritative.
PJ
