The data is based on the number of Android devices that accessed Google Play within a 14-day period ending as on 01st Jan, 2013. The data reveals that the combined share of Jelly Bean 4.1 and 4.2 accounts for 10.2% of android devices while the Ice Cream Sandwich (Android 4.0) accounts to 29.1% of devices.
Devices running the prior versions of the android OS like Donut, Eclair, Froyo, Honeycomb are all well under 10% each and the one being run the most is Gingerbread with a distribution share of 47.4%.
If you develop your application for the version that is at the very top of the chart, then your application is compatible with 100% of active devices (and all future versions), because all Android APIs are forward compatible. Or, if you develop your application for a version lower on the chart, then it is currently compatible with the percentage of devices indicated on the y-axis, where the line for that version meets the y-axis on the right.
Jelly Bean 4.2 has seen considerable features like Photo Sphere that can capture 360 degree images and create immersive Photo Spheres which can be easily shared on Google+, Multi-User support, switching between accounts, mirroring phone screen to HDTV with wireless adapter and others. Apple meanwhile has also launched iOS6 to its credit which is second to none. The rivalry of Google and Apple OR Android vs iOS is well documented and will be interesting to see how the the face-off between them looks like in 2013.