What is the differences between distributions, the OS and code names (stable, testing) in linux and what order is that listed in /etc/*release?

Overall, I'm trying to understand the differences between distributions, Operating systems and release code names (stable, experimental, testing) for Linux and how can those are listed in release files. I know some are based on toy story characters and I'm using Ubuntu but I'm not sure as to what the different names mean. Not where to look for them. When I run lsb_release it says ubuntu 14.04.3 but if cat /etc/debian_version it says jessie/sid.