The newest stable version is 0.9.1. The newest version (including RC and devlopment) is 0.9.1.
Stable releases were tested as Development and RC by a large number of users before release.
Release candidates are polished development builds being prepared for stable release.
Development versions are most up-to-date but may contain bugs (and nuts).
Editor Upgrades adds new object to 2D Editor and Map Builder. Some upgrades contain their own addons, other just add compatibility with Map Builder to third party AddOns (like JBAD).