Appendix E. The Linux 2.6 Kernel
The Linux 2.6 kernel was released on December 17, 2003. A major release of a Linux kernel is not an everyday occurrence: The last kernel, Linux 2.4, was released in January 2001. This appendix lists features that are new to the 2.6 kernel.
Linux kernel revisions alternate between stable and unstable versions: 2.4 was the previous stable version, so 2.5 was the development branch, which later became 2.6. For each of the major revisions, there is a series of minor revisions. Usually, minor revisions do not contain major changes, although one minor revision to the 2.4 kernel replaced the entire virtual memory subsystem, a major part of the kernel.
See www.kniggit.net/wwol26.html if you want more information on the Linux ...