At any given time, there are several “stable” versions of Linux, and one “development” version. Unlike most proprietary software, older stable versions continue to be supported for as long as there is interest, which is why multiple versions exist.
Linux version numbers follow a longstanding tradition. Each version has three numbers, i.e., X.Y.Z. The “X” is only incremented when a really significant change happens, one that makes software written for one version no longer operate correctly on the other. This happens very rarely — in Linux’s history it has happened exactly once.
The “Y” tells you which development “series” you are in. A stable kernel will always have an even number in this position, while a development kernel will always have an odd number.
The “Z” specifies which exact version of the kernel you have, and it is incremented on every release.
The current stable series is 2.4.x, and the current development series is 2.5.x. However, many people continue to run 2.2.x and even 2.0.x kernels, and they als o continue to receive bugfixes. The development series is the code that the Linu x developers are actively working on, which is always available for public viewing, testing, and even use, although production use is not recommended! This is part of the “open source development” method.
Eventually, the 2.5.x development series will be “sprinkled with holy penguin pee” and become the 2.6.0 kernel and a new stable series will then be established, and a 2.7.x development series begun. Or, if any really major changes happen, it might become 3.0.0 instead, and a 3.1.x series begun.