All WP8 phones to be updated to WP10, says Microsoft

Windows phone was launched in November 2010 brought metro UI (inspired by Zune HD) to smartphones. the home screen consists of the "live tiles" which update himself and display information. WP OS was launched with version 7 which was in continuance with the old windows mobile OS. which was updated with v7.5 (also known as mango) in may 2011. In the year 2012 another update (tango) was released to lower hardware requirements of the OS so that the OS can run on phones with less powerful hardware.

later on October 2012, Microsoft released WP8 it replaced the previous windows CE-based architecture with one based on windows NT . the WP8 added many software improvements and support for better hardware like multi core processors & high resolution screens.

with the launch of WP8,Microsoft announced that the WP7 phones would not get updated to WP8 which was very disappointing for WP7 users. later Microsoft released a small update for WP7 phones which were updated to v7.8 and brought some features from WP8(as they were not upgradable to WP8 due to hardware limitations)

Microsoft on April 2014 launched WP8, it brought many new features like notification centre also known as action centre, IE11, tab sync, separate volume controls, camera improvements. and ability to apply skin on top of live tiles. one of the major features was Cortana a voice assistant which replaced Bing search. which learns you preferences and gives you tailored answers and recommendations.

Now Microsoft is all set to upgrade. they previewed the Microsoft windows v10 for computers. and recently a tweet from Microsoft confirmed that they are working on windows 10. but Microsoft made sure that you didn't get behind and said that all the phones running WP8.1 will get the WP10 update.
they havent announced any features or the possible release date.