Today Google took steps to get all of its entertainment ducklings in a row. Google Play is the new umbrella that will cover the company's Apps, Movies, Music, and Books offerings. It'll cover both desktops and Android phones. Think iTunes for the Android set.
There's nothing really new here in terms of functionality, it's more of an aggregated user interface thing. Starting today, you'll just go to play.google.com for all of your Google-based media. It looks a lot like Android Market does now, but it's a bit more intuitive. On the left you have links to Music, Movies, Apps, and Books, where you can search for and purchase all of those things, and on the top bar you'll have My Music, My Movies, My Apps, and My Books, where you can peruse the library of stuff you already own.
For Android users, the old familiar Android Market will be morphing into the Play Store. Music will be Play Music, Books will be Play Books, and so on. Again, as of now there doesn't seem to be anything new feature-wise, but they're working on making the user experience more cohesive. Current Android users will soon be getting a notification about these changes.
Starting today(ish, they typically roll things out over a few days) when you're logged into Google services on the web, you'll see a Play tab at the top of your window next to Calendar, Documents, Search and the others. Clicking that will take you right into your Google Play homepage.