Release Notes are a list of the new features and bug fixes added to each new version of Second Life. Second Life consists of the following two distinct pieces of software that work together:
• The viewer, which is the program you downloaded from secondlife.com and installed on your hard drive.
• The simulator, which runs the 3D simulation that you walk through and experience while in Second Life.
Periodically, Linden Lab will upgrade the Viewer and the Server to new versions, but not necessarily at the same time. Because they are released separately, there are distinct Release Notes for each version released (see What do the numbers in a release version mean?)
Release Notes for Second Life Release Candidates and Second Life Beta Viewers can be found in the links above.
Note: The independent website secondlife.wikia.com also keeps an archive of Release Notes, especially for very old versions.