What is YouTube Music?

YouTube Music is a music streaming service from YouTube.

Your music and artwork will be automatically combined to create an "Art Track," which is a video on YouTube that displays your artwork & plays your song.

You're paid for each monetized play of your music. A "monetized" play is when YouTube displays an ad on the video, or when a YouTube Music paying subscriber listens (in which case you're paid, pro-rata, based on how much he's paying and what percentage of the time he listened to your tracks). 

YouTube Music is currently available in 5 territories: US, MX, KR, AU & NZ, and will be available anywhere YouTube has a subscription music service as new territories are supported. If you're trying to play YouTube Music Art Track videos from outside of these supported territories, you may see a message that the content is not available.

Here's some more information: 

http://youtube-global.blogspot.com/2014/11/youtube-music-2014.html

http://www.theverge.com/2014/11/12/7201969/youtube-music-key-new-subscription-service