Release notes for the Second Life Viewer and simulator are kept on the Second Life Wiki.
Release notes list features and bug fixes for each new version of Second Life. Second Life consists of the following two distinct pieces of software that work together:
The Viewer, the program you downloaded from secondlife.com and run on your computer. See Release Notes on the wiki.
The simulator (server), that runs the 3D simulation you experience while in Second Life. See Server Release Notes on the wiki.