Steam is a sort of online store for games. You can buy and download games through steam, without having to go out of your house. Contrary to what some people will tell you, Steam games can be played offline. Steam also functions as a community, and it has a chat feature, allowing you to chat with friends, even if you're not in the same game. You can also add non-Steam games to your Steam library, so the chat feature can be used with them as well.
There's also a Subsim Steam group
