Back when FreeBSD 2.0.5 was released, FreeBSD development branched in two. One branch was named -STABLE, one -CURRENT. FreeBSD-STABLE is intended for Internet Service Providers and other commercial enterprises for whom sudden shifts or experimental features are quite undesirable. It receives only well-tested bug fixes and other small incremental enhancements. FreeBSD-CURRENT, on the other hand, has been one unbroken line leading towards 5.0-RELEASE (and beyond) since 2.0 was released. If a little ASCII art would help, this is how it looks:
2.0 | | | [2.1-STABLE] *BRANCH* 2.0.5 -> 2.1 -> 2.1.5 -> 2.1.6 -> 184.108.40.206 [2.1-STABLE ends] | (Mar 1997) | | | [2.2-STABLE] *BRANCH* 2.2.1 -> 2.2.2-RELEASE -> 2.2.5 -> 2.2.6 -> 2.2.7 -> 2.2.8 [end] | (Mar 1997) (Oct 97) (Apr 98) (Jul 98) (Dec 98) | | 3.0-SNAPs (started Q1 1997) | | 3.0-RELEASE (Oct 1998) | | [3.0-STABLE] *BRANCH* 3.1-RELEASE (Feb 1999) -> 3.2 -> 3.3 -> 3.4 -> 3.5 -> 3.5.1 | (May 1999) (Sep 1999) (Dec 1999) (June 2000) (July 2000) | | [4.0-STABLE] *BRANCH* 4.0 (Mar 2000) -> 4.1 -> 4.1.1 -> 4.2 -> 4.3 -> 4.4 -> ... future 4.x releases ... | | (July 2000) (Sep 2000) (Nov 2000) \|/ + [5.0-CURRENT continues]
The 2.2-STABLE branch was retired with the release of 2.2.8. The 3-STABLE branch has ended with the release of 3.5.1, the final 3.X release. The only changes made to either of these branches will be, for the most part, security-related bug fixes.
4-STABLE is the actively developed -STABLE branch. The latest release on the 4-STABLE is 4.6-RELEASE, which was released in June, 2002.
The 5-CURRENT branch is slowly progressing toward 5.0-RELEASE and beyond. See What is FreeBSD-CURRENT? for more information on this branch.