From Second Life Wiki
Channels are just groupings of versions. We use channels to track and supply updates for First Look viewers separately from Release and Release Candidate viewers. Passing this argument changes many of the strings describing the viewer, such as the installation location and application name, as well as passing the -channel argument to the viewer on startup.
See Channel and Version Requirements for a detailed explanation and official policy on how Linden Lab deals with channels and versions in Second Life.