Steam is a gaming platform as mentioned. They have been around for a number of years and have sales regularly as well as free to play games, and support Windows, Mac, and Linux. Their increased support for Linux is one of the reasons why I like their platform. They also have chat, similar to MSN, Yahoo, and AOL messengers of yore. I have friends who, like myself, don't use other forms of social media so I can only get in contact with them easily through there. I avoided it for a number of years before caving in because I couldn't contact people for weeks at a time.
They also have other software and videos for sale, as well as voice chat and other stuff they are expanding on, but I haven't used any of that yet.
Michael Cox wrote:It can get annoying at times, as I have unreliable internet access and it usually wants you to be connected to play.
You can use Steam in Offline mode to play single player games. Obviously this means online features like playing and communicating with others is disabled, but if your connection gets flaky at times and you just want to relax with a single player game then it works. The only thing to be careful about is when you do go back online it will have a discrepancy with the Steam Cloud version of your game saves versus your local version, so if you pick the wrong one it can undo anything you did offline. It can also accidentally wipe out your save and leave you with nothing. If you have an important game save, you can look up how to find the save location for a particular game and back it up regularly.