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 the US, UK, Germany, France, Spain, Italy, Russia, Canada, Norway, Ireland, Finland, Norway, Sweden, Australia, New Zealand, Mexico, and South Korea, 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