The Android Market has been around for a while now, but Google has begun using it for more than just Android apps; they have added Music, Movies and Books over the last year. So, in an effort to unify things a bit and get a less ambiguous name that describes the service, Google is re-branding the Android Market as Google Play. The website is active now, and the Android Market app on your devices will be replaced with the Google Play Store app. You can also expect to see updates today for the Books, Music and Movies applications.
For more, check out the Google Play website which is available now.