OS versions
The OS is based on the OpenEmbedded build framework for embedded linux. Starting from a base version of the linux kernel, various customisations are applied to create an OS which runs on the Pandora. From an OS version, small changes will be incrementally developed, and these are released by the OpenPandora team as Hotfixes - PND files which can be used to update an already installed OS.
After a while, there may be significant changes which justify or require a larger change, and that will prompt a new version of the OS.
Zaxxon
The first OpenPandora OS release. Hotfixes 1-6 are released (10 October, 2011). it is likely that HF6 will be the final Hotfix for this release.
The latest version of Zaxxon can be downloaded here. For betas of newer hotfixes, check the Beta testing forum.