From the MSDN blog post:
The API is a set of easy to use REST methods, including CORS, JSONP and Flash cross domain support. The complete documentation is available here.
Music is first and foremost about content. We've opened access to our 38 million track catalog in 23 countries. We let you search our catalog for artists, albums and tracks. Once you've found what you're interested in, you can lookup extra information for that piece of content, such as an artist's albums. Want to make content more appealing in your application? We also give you access to an extensive set of artist and album images. We also provide deep-links into existing Xbox Music applications on most of our platforms.
Taking it a step further, you get access to user authenticated features of Xbox Music. User authenticated features will at first be restricted to members of our pilot program. Read the subscriber's music library and playlists, add new music content or create new playlists. Curating content to fit a user's taste is complex. With access to a user's personal collection, you can tailor your experience to better meet his needs in your experience. Everything you do to the user's collection and playlists will roam seamlessly to Xbox Music first party apps as well as other third party apps like your own.
I'm sure the question you're all burning to ask is how to get access to actual music in your own applications. The good news is that we're providing full access to 30 second MP3 previews for our entire catalog. They're served as HTTP progressive downloads, making it simple for you to use them. If you want to push integration further, we allow access to full track streaming for Xbox Music Pass subscribers.
You no doubt know that authentication is often a tricky bit when integrating with third party services. We're providing simple to use single sign-in integration with Microsoft Account for user authenticated scenarios. For other features, you only need to sign-up to the Xbox Music offer on Azure Data Market to get an access key.
As an incentive to get going, there's an affiliate scheme:
On top of service features, we're also launching a generous affiliation offer. Every user you redirect to the Xbox Music application can earn you money on content purchases and Xbox Music Pass subscriptions. You currently will earn a 5 percent share on purchases and as the Xbox Music pass is at the core of our service, 10 percent on all music pass payments for the lifetime of the subscription. In the US for example, that's one dollar, per user, per month!
If you're a developer and want more information, see the other links from the original MSDN post.
Note that we're all (as consumers) still waiting for an official client for keeping your general music in OneDrive. This is something that only MusicConnect has tried so far, but it seems that with the Xbox Music APIs opening up, the whole WP music scene is about to get more interesting.