Today Valve updates their Steam client to improve stability and add some new convenient features. The update includes fixes to the general Steam client, Big Picture mode and Linux operating systems.
Most notable in the update is the ability to download games and updates while playing other games. Before the update, game patches and downloads paused even if you played a game's single player modes. The update brings fixes and improvements to the client itself. Any cloud syncing problems, slower download speed and Steam overlay bugs should be resolved with this update.
The timing coincides perfectly with the Steam Autumn sale, which undoubtedly added many games to Steam users' every-expanding libraries. The download and play feature should help players clear their backlog without any time wasted waiting for game downloads.
For a full list of fixes in the update, Valve outlined the update here. The Steam Client update will automatically download when you launch Steam.