Please only use this tag if you feel your question is specific to Android 5.0 -- the 10th publicly named Android release which debuted with the Nexus 6 and Nexus 9 in November 2014.
Android 5.0 Lollipop was unveiled on June 25, 2014 during Google I/O. It became available to select Nexus devices as an official over-the-air (OTA) update on November 12 2014.
'Material Design' is one of the prominent changes in the Lollipop release. Other changes include allowing notifications to be accessed from the lockscreen, and to be displayed within other apps as banners across the top of the screen. The Android Runtime (ART) officially replaced the Dalvik runtime for improved application performance. Other features include: