The app for iPhone and web will use Twitter activity including Tweets and engadgement to detect the most popular tracks and emerging artists. It will also bring in artists' music-related to Twtiter activity. You can go to profiles to see which music they follow and listen to other songs by the artists.
Currently songs in Twitter #music come from iTunes, Spotify and Rdio. By default you will get previews from iTunes when exploring, but subscribers to Rdio and Spotify will be able to login to their accounts and listen to full tracks. Twitter says they are working to add more music service providers.