YouTube Music is a Music streaming service operated by Google.
It is a Spotify, Deezer, Apple Music rival, offering same features (All Catalogs, Ad free, Offline experience)
The exhaustive list of territories where the app is available can be found here.
Users will be able to access YouTube Music through a dedicated app on iOs and Android and a brand new desktop player that is designed for music : https://music.youtube.com
The content of YouTube Music, will be accessible on YouTube Standard.