PeerTube is decentralised by most useful definitions of the word. Anybody can run their own PeerTube instance, and the instances can follow (or “connect”) to each other to form a larger whole. This is not an option you have with YouTube.
When you watch a video on PeerTube the data will come from the instance hosting the video, any other instance that follows that instance and has redundancy enabled and other people watching the video at the same time as you do.
See the new features at PeerTube 4.0 Free Video Platform Comes More Powerful Than Ever
An overview of the new version of PeerTube, a software solution to create alternative platforms to YouTube.