Official Vs Unofficial? Whats the difference? topic

I look through the Dev forum here and I see a host of lollipop builds, some claiming to be official, some claiming to be unofficial, most seem to be derivative of CM12.

So Just what is the difference between an Official ROM and an Unofficial ROM?

Surely the only "Official" CM12 ROM is from https://download.cyanogenmod.org/?device=t0lte (for the n7105)?