Life Cycle

This is the typical life cycle

LTS Releases
LTS Releases (or Long-Term-Support) is when your version lasts for a year. These normally only get feature updates, along with driver. The first LTS will be 19.06 - due for release in June 2019 and support ending June 2020.

STS Releases
STS (or Short Term Support) lasts around half a year. These only feature kernel updates.

Pre-Release (PR)
These are testing builds and are not to be released into the wild without permission. They receive no bug fixes/optimization as it is PR by PR.