Steam’s new User Interface that has been in beta for a few months has finally gone live for everyone across the approximately 25 million accounts that use the service. According to Valve, the new interface “enhances the way users buy and play games, providing more info about the things that matter – getting games, playing them with friends, and staying current in the gaming world.”
Now, the improvements are plentiful so I would recommend you check out the list here if you are interested in knowing every last detail. Most importantly: There was a major overhaul to the client and store and the largest overhaul since the client launched in 2003.
To receive the update, simply start Steam as you normally do and it will automatically update for you.