The Number of the Kernel
Linux kernel releases are numbered. By convention, EVEN releases (1.0, 1.2, 1.4, ...) are "official", while ODD releases (1.1, 1.3, ...) are beta/developer versions. A third number gives the "fix" level. Older distributions contain v2.0.xx, but v2.2 was released in February, 1999 and can usually be downloaded as a replacement. The newest distributions (e.g., SuSE 6.1, RedHat 6.0) contain v2.2.xx as standard.
Several other programs are adopting the same convention. For example, GIMP v1.0 is official and v1.1 is beta.
Note that the Linux kernel version has no relation to distribution numbering - for example, SuSE v5.3, Red Hat v5.2, Debian v2.1, and Slackware v3.6 all contain the 2.0.35 or 2.0.36 kernel. Always read the fine print.