According to a blog post on Android Dev Blog, 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.
Rest of the work is done by Android Market, so when a user downloads an app, Android Market chooses the right APK to deliver based on the characteristics of his/her device.
Finally, App devs need not have different app listings for tablets/phones version of an app or even based on resolution or compatibility with a specific SoC. This will certainly clear out a lot of mess for developers and will make it easier for them to track their app listings. More details here.
