Google has released Android 5.0 Lollipop on October 2014 with a major change in UI (User Interface) design with lot of additional features and bug fixes. To fix few bugs in Android 5.0, on December 2014 Google released 5.0.1 and 5.0.2. Preceded by 5.0.2, Google officially released Android Lollipop 5.1 on March 09, 2015 with new enhancements and security features. But on February 2014, Android One was launched pre-loaded with Android 5.1 version only in Indonesia and Philippines.
Enhanced Quick settings menu to control and manage Wi-Fi and Bluetooth connections from inside the Quick settings menu instead of opening in a separate Wi-Fi or Bluetooth settings window.
Multiple SIM Support: Updated Android 5.1 to support multiple SIM switching. For example: If your mobile has more than one SIM slot, then you can easily switch the SIM cards instead of changing it physically.
In Android 5.1, Google released a major security feature to protect lost or stolen device. If a device was lost or stolen, then it will remain locked until signing in with the google account which was added to that device. It was implemented in such a way that even factory resetting won’t help to unlock the device.
HD Voice Calling: Users can experience High Definition (HD) voice quality while making calls in between Android 5.1 mobiles.
However lots of issues are reported by android users, Google missed to provide fix for several reported bugs. Hope they will continue to provide in future releases.