Downloadable straight from Google Play, you can boot up the app, connect to your host machine over your wireless network, then get to playing all of your PC titles. While it is not to be confused with the Steam Link gadget, in practice it more or less does the same thing where users can stream their games on Steam to their mobile devices via the app so that they can play PC games on their phones.
As long as your PC and phone are on the same Wi-Fi network, Steam Link will be able to beam games to your device. Valve Senior Software Engineer Sam Lantinga told Variety that "if you have a powerful rig, wired network and very good client device, it's possible to stream at 4K 60 fps". When it does launch, it will reportedly support any device running iOS 10 or newer, and any Apple TV running tvOS 10.3 or above. The option of streaming via LTE connection has not been provided in the beta version.
Only the Android version of Steam Link has been released for now.
Compatible with devices running at least Android 5.0, the application is neatly designed. "You can go into the advanced streaming settings and tune streaming resolution and bitrate for the best experience in your setup". In that case, you will still need to have your PC connected via an Ethernet cable to your router, and make sure that the Android TV is connected via Ethernet to the same router.
You'll also want to use a Bluetooth controller for the most part: touch input is recognized, but it's not ideal, and most games don't support it for actual gameplay (even if it works in menus and such).