There have been cloud gaming projects that you let you play remotely from your smartphone. However, Steam has added an interesting local multiplayer feature that expands its reach over the Internet. It is called Steam Remote Play.
Normally, for a game that supports Local multiplayer co-op for up to 4 players, you need to be present in a single location. Not anymore. With Remote Play, you can jump right into a game that supports a local multiplayer mode, remotely.
You can even share your input keyboard/controller with your friend. To make it work, you just need to have Steam installed on your device. As well as the ability to play local multiplayer games over the Internet, the best part is, you are not required to purchase the game or own it separately.
Only the host (with a PC) needs to own the game on Steam and all they have to do is invite their friends via Steam Chat to join in for local multiplayer action over the Internet. It works similar to how you invite friends over to join your game on Steam. This is a pretty nice addition to steam. If you love coop games this is for you.