The Steam Link is a device that allows you to utilize the In-Home Streaming feature of Steam away from your PC. While the Steam Link will be running some form of Steam OS, it'll likely be bare bones (as it doesn't have any dedicated storage for games) in order to focus on in-home streaming.
With that said... If you add a web browser (like Chrome or Firefox) into your Steam library, you can watch Netflix & Youtube through Steam Link (as it'll be running on the connected PC... but the I/O steams will be going to the Link) as a result. If you really want to lazy-hack the Link, you can pretty much add any non-Steam application on your PC into your library & have access to it through the Steam Link... emulators, media players, office suites... you can go crazy-stupid with this!
Just be aware that the Steam Link is more of a "terminal" to the PC(s) that are on your network. If somebody else is using the computer, you'll likely run into problems using the Steam Link at the same time as them... since you're accessing the same computer.
Hope this helps!