Lamport’s last version of LaTeX (LaTeX 2.09, last updated in 1992) was superseded in 1994 by a new version (LaTeX 2e) provided by the LaTeX team. LaTeX 2e is now the only readily-available version of LaTeX, and draws together several threads of LaTeX development from the later days of LaTeX 2.09. The “e” of the name is (in the official logo) a single-stroke epsilon (ε, supposedly indicative of no more than a small change).
LaTeX 2e has several enhancements over LaTeX 2.09, but they were all rather minor, with a view to continuity and stability rather than the “big push” that some had expected from the team. LaTeX 2e continues to this day to offer a compatibility mode in which most files prepared for use with LaTeX 2.09 will run (albeit with somewhat reduced performance, and subject to voluminous complaints in the log file). Differences between LaTeX 2e and LaTeX 2.09 are outlined in a series of ‘guide’ files that are available in every LaTeX distribution (the same directory also contains “news” about each new release of LaTeX 2e).
Note that, now, LaTeX 2e is “feature frozen” (the only allowed changes come from bug reports); this, too, is in pursuit of stability, and is a driving force for many of the efforts to contribute LaTeX packages.
This answer last edited: 2011-08-19