Note: The original post was written before the official announcement. It remains here unmodified as a historical reference. For updated information, see the bottom of the post.
It is expected that Google will reveal Play Games at Google I/O in two days. What is Play Games? It's Google's new service for Android that adds achievements, leaderboards, cloud saves, and more. It hasn't been announced and can't be used yet. But there's still a fair bit known about it. Let's take a look.
Play Games will be implemented through Google Play services. There are two components to Google Play services: the Google Play services APK and the client library. The APK is automatically installed and updated by Google Play. The client library is included with apps that interact with Play Services. Play Games will require updated versions of both.
The updated Play services client library first showed up in the MyGlass app. Here's what Android Police found inside.
These classes would be used for real-time multiplayer and turn-based multiplayer.
Also supported are leaderboards, invitations, and lobbies.
With all this other stuff, it would be a shame if Google forgot about achievements. Fortunately, they didn't. Look, right there!
Filenames are boring. What else is there? What about the update to the Google Play APK? Here's what Android Police found.
The Play Games icon looks like the Games icon used in the new version of the Play Store, with the standard Google Play pattern applied to it.
Activities for achievement lists, leaderboards, invitations, and more are included. It is unclear whether developers will be able to implement their own custom UIs for these functions or whether these prebuilt activities must be used.
Play Games accounts are Google+ accounts, so your real name will probably be visible to everyone you play with. In the Play Games settings you have the ability to control which of your Google+ circles can send you notifications. You can also mute games, which I suppose is a way to prevent certain games from sending you notifications.
Leaderboards show the top ranking players of all time, weekly, and today, and you can see how you rank against your friends.
Games can synchronize save data through the user's Google account, keeping your saves up to date on multiple devices.
This is the "achievement unlocked" thingy. I don't know whether there will be a sound when it pops up, so feel free to speculate.Look for more information on May 15, when Google Play Games is expected to be announced.
UPDATE: Google Play game services has been announced at the Google I/O 2013 keynote. Polygon reports that it will also be available for iOS and web.