What is the FreeBSD-STABLE concept?

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 -> 2.1.7.1  [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.