This feature was announced back in May at the Google I/O event in San Francisco, but only now it became official. The multiple APK support, is now available, a new publishing option in Android Market for those developers who want extra control over distribution.
“With multiple APK support, you can now upload multiple versions of an APK for a single product listing, with each one addressing a different subset of your customers. These APKs are complete, independent APKs that share the same package name, but contain code and resources to target different Android platform versions, screen sizes, or GL texture-compression formats. When users download or purchase your app, Android Market chooses the right APK to deliver based on the characteristics of the device.”
To support this new capability, Google Devs updated the Developer Console to include controls for uploading and managing APKs in a product listing.
If you want to learn more how this feature (multiple apk support) really works read the developers documentation.
source: Android Devs Blog