All operating systems have a kernel. The kernel is the heart, the life-blood, the core of an operating system. Everything else is just there to make the operating system worth using. The kernel provides low-level services, such as memory management, basic hardware interaction, networking, and security.

The Linux kernel version numbering system works as follows:

z.y.x

z = Major version  (we've had 0, 1, and 2 so far)
y = Minor version
x = Revision level

Development of the Linux kernel uses a 'two-tree' system. There's the development tree and the stable tree. If y is an odd number, it's the development tree. If y is even, it's the stable tree.

At the time of writing, 2.3.28 is the latest development kernel and 2.2.13 is the latest stable kernel.

When Linus (the kernel's creator) decides the 2.3 kernel is ready to become the next stable tree, the 2.3 tree will become the 2.4 tree, and work will commence on the 2.5 tree.

All new (and therfore experimental) features enter the development tree -- making parts of the development kernel unstable and unsecure. For this reason only kernel developers, the instatiably curious, and those that desperately require some feature only available in the development tree should go near it.

If the change to the development kernel is a bug fix and the bug is also in the stable kernel, then after some testing the fix will also be made to the stable kernel.

Suffice to say, Linux distributions never use a development kernel.

The 2.0.x series was officially discontinued as of June 1999, and we'll soon start to see Linux distributions featuring the eagerly awaited 2.4 kernel (approx. Feb. 2000).