As announced back at Google I/O, Android developers now have the ability to upload multiple APKs (individual package files) under the same listing in the Android Market. When you download an application that has multiple APKs associated with it, the Market will intelligently pick the APK that is most compatible with your device.
This gives developers an alternate method of supporting a wide variety of devices (say, phones and tablets) without stuffing complicated logic and graphical assets into a single APK. If used right, the new technique, which is completely transparent to end users, will result in smaller app sizes and eliminate the need for separate Market listings (case in point: Qik).
Source: Android Developers Blog