Windows 10 Mobile is the latest iteration of the Windows operating system, now unified with the Windows Phone OS. It was released as an opt-in Technical Preview 2 (build 9941) on February 12, 2015 to a subset of current Lumia phones, including the Lumia 635, Lumia 730 and Lumia 830. It has since expanded to include almost all of the current Lumia range.
Windows 10 Mobile is the first attempt by Microsoft to unify their desktop, tablet and phone operating systems into a single OS. As such, Windows 10 on the phone shares many of the same features as its desktop version, including the same kernel, UI elements, menus, Settings, and even Cortana. It will also support Continuum on some, newer smartphones, allowing a desktop-like experience by plugging the phone into an external monitor.
With the exception of 512MB devices, all current Windows Phone 8 and 8.1 phones should be able to upgrade to Windows 10 for free, although carrier approval processes may slow down its rollout. The Windows Insider program, now expanded to phones, allows customers to bypass the carriers and install the new operating system on their phones directly.
Further information on how to enroll in the Windows Insider program for phone can be found here.