"Starting next week, as a Google Play Music subscriber, you'll get free and complete access to the YouTube Music Key beta, a new service from YouTube where you can watch ad-free music videos, and keep the music playing in the background or when you're offline. The Google Play Music app will also include ad-free music videos alongside select tracks," informed Google.
This means that Play Music subscribers will be able to use Music Key starting today. It's worth pointing out that Music Key will only be available in the US, UK, Ireland, Spain, Italy, Finland and Portugal.
Music Key and Play Music All Access are complementary services. Music Key uses Play Music data to show recommendations and metadata, Play Music's catalog is enhanced by adding YouTube's huge music library. Play Music's subscription service offers more value, while Google manages to use YouTube's strengths to make Play Music more popular.