Fragmentation is the term used to describe all the variations available on the Android platform. Android has many manufacturers and models, unlike iOS where there is only one manufacturer, and they only have 3 (or 4) different models: iPhone, iPod, iPad (and iPad Mini).
The Android Dashboard is a great resource for tracking the adoption of different versions of the Android OS. It is updated monthly and also covers screen sizes and densities and Open GL versions.
Another great resource is OpenSignal fragmentation report. It shows market share by model and manufacturer, as well as some different ways of looking at the information in the Android Dashboard, as well as screen size. It appears to be updated annually.
I didn’t realize Samsung had almost 50% of the Android market, and I expected HTC to have a larger share. No wonder Apple feels threatened by Samsung. Noticeably missing from their list is any Amazon Kindles or Barns and Noble Nooks. That could be because they do not access the Google Play Store by default. I didn’t see the method of data collection.
Fragmentation is a bit of a headache for the app developer because there are so many different variations to take into consideration. The flip side though, is the flexibility of Android has really lead to its mass adoption. It works in both the ultra high-end luxury devices as well as the low-end economy devices. So while it is difficult to get an app that works on all devices, you still have a lot of possibilities to choose from.