"CyanogenMod tries to build a new, fresh "nightly" version every 24 hours for each of the devices it supports, which includes the most up-to-date changes to the source code, provided from all over the Internet. Of course, the nightly builds may also contain newly-introduced bugs, but hey- if you feel adventurous, you can help make CyanogenMod better by trying these experimental builds and reporting back bugs to the developers."
Team Osiris' build I believe was specifically made for Boost Mobile Motorola G owners. It wasn't off the unified device tree.
An UNOFFICIAL build is any build of CyanogenMod not built by Cyanogen.