After Android L aka Lollipop its time to move on with Android M courtesy Google.
Sundar Pichai, senior vice president of products at Google said-> the company had "gone back to basics" with the new version of Android. While Android Lollipop introduced a brand new design and aesthetic, Android M is bringing some much-needed stability and usability improvements, which might not be as immediately arresting, but may prove to be significant long-term additions.
Android L has many bugs {unstable} which can be overwritten by this OS along with some security features - according to Google
HTC has already announced two of its flagship phone HTC One M9 and One M9+ along with M8, to get this update. Also some other mobiles namely MOTO X, Samsung Galaxy S4, S5, Note3, Note4, Sony Xperia Z1 will also get this update.