I first wrote about the various versions of the Legion back in November. I wanted to try to re-organize my thoughts on the reboots in preparation for some upcoming blog entries, and in the wake of Infinite Crisis (both the Secret Files story and the last couple issues of the miniseries), I'm sure I'll have more to say. For consistency's sake, I've labeled them like computer software versions since we're all familiar with that, but nobody else uses these labels (yet).
(For a vastly bigger look at the different versions of the Legion, Don Sakers has cataloged every variation of the Legion, from Ferro Lad II to Bruce "Superman" Wayne, over at the Alternate Versions page. Bet you didn't remember all of them!)
LSH v1.0, 1958-1986
The original series up through the Crisis. Superboy, Supergirl, yada yada yada. (Don's A.0.00 to A.0.30)
LSH v1.1, 1986-1990
In the wake of the original Crisis, Superman was rebooted, which retconned Superboy out. The easy way out was to just ignore Superboy in current continuity, but that got messy when the Legion (who knew Superboy) met the new Superman (who was never a Superboy), so they invented the Pocket Universe. That allowed them to keep *a* Superboy (but didn't explain how the Legion met Superman, since the PU Superboy died shortly thereafter). Still, it wasn't TOO messy, as long as you didn't think about it much. Then Giffen and the Bierbaums came along and jumped us Five Years Later into a dystopian future. Since the majority of the backstory wasn't retconned away, this gets a "point" upgrade from 1.0 to 1.1. (Don's A.2.50)
LSH v2.0a (alpha), 1990
Very early in the series, a battle with the Time Trapper caused a brief reboot into the "Mordruverse", which showed what happened when Mordru ruled the universe. That story ended with the Mordruverse rebooting. This was only a single issue. Because that timeline was unstable, this version that totally reworked the Legion was scrapped in the "alpha" phase and wasn't allowed to continue. (Don's A.2.60)
LSH v1.2, 1990-1994
After the Mordruverse rebooted, the DCU Legion went back to the most recent version (v1.1) some changes were made ("it's not a bug, it's a feature!") - there were never a Superboy or Supergirl, instead there was Valor (formerly Mon-el) and Andromeda, who were retconned in place of the Kryptonian duo. Still, it was mostly the old continuity and characters. Along the way, a group of "kid Legionnaires" called the SW6 Batch showed up. This timeline ended with Zero Hour. (Don's A.2.70)
LSH v3.0, 1994-2004
Post-Zero Hour, the Legion was rebooted with a new continuity that started from scratch, although it had parallels with the old series. You didn't have to have read the original stuff, but it helped. In a sense, the old version was discontinued and the Legion was rewritten from the ground up with a new user interface. As time went on, it diverged more and more. It started with a happy and light future, led by Roger Stern, Tom Peyer, and Mark Waid, and then took a drastic turn to darkness once Abnett and Lanning came on. They killed some characters and introduced new ones. This series ended as the Legion was lost in space due to a battle with the Fatal Five (Hundred) along side the Teen Titans in the Titans/Legion Special. Since LSH v2.0 never left the alpha stage, and since this version did not build off of v2.0a, I gave this the v3.0 designation. (Don's B.2.20)
LSH v4.0, 2005-present
Probably due to sales and an impression of a complicated user interface, the Legion was rebooted again (or the "threeboot" as some are calling it), with Waid and Kitson. Again, it starts from scratch and has some characters the same as the old series, and you don't need to have read the old series in order to understand the new one. The ideas and concepts are similar, and this version tries to do the same sort of stuff as the older versions, but with a different user interface written by a trusted name. With but each reboot (or version number increase), whatever is the current version of the Legion moves further and further away from previous versions.