Android Operating System Information

Android OS Project home page:  https://source.android.com/

            This is where developers will want to go for sure to understand what's been updated.

Social Compare - Android Versions: http://socialcompare.com/en/comparison/android-versions-comparison  

            This is a great site to get a quick view of updates.

                        This tracks all updates.

                        This separates User stuff from Developer stuff