In Short
Windows 10 is an operating system from Microsoft Corporation for servers, desktop PCs, laptops, tablets, phones, and other connected devices (i.e. Internet of Things). The OS was formally announced on September 30, 2014, in San Francisco with Microsoft's Joe Belfiore and Terry Myerson on stage. It was released worldwide on July 29, 2015.
Windows 10 for Phones was released as a Technical Preview on February 12, 2015 to various Lumia phones. The OS had the internal codename 'Threshold'.
A Technical Preview for the OS was launched on October 1, 2014 by Microsoft for users to try an early version of Windows 10 for feedback. On January 21, 2015, Microsoft revealed the consumer side of Windows 10, including its plans for mobile and gaming. The final unveiling of Windows 10 took place during Microsoft's Build 2015 conference.
Windows 10 'skips' Windows 9 for its name for various reasons, including it being such a big update that it moves beyond just a single number upgrade.
Windows 10 gives users more choices between using the Start Screen (ideal for tablets and touch-screen devices) and the Start Menu (better suited for PCs and traditional laptops). A forthcoming feature called 'Continuum' lets the OS automatically switch between tablet and desktop mode depending on the presence (or absence) of a mouse and keyboard. Users can also override any settings, making the default Start Menu or Start Screen their choice.
Other features in Windows 10 include multiple desktop support, Modern apps are scalable on the desktop, new Start Menu (combo of traditional Start Menu plus room for Windows Apps), new task view button, new snapping options for apps, and improvements to file explorer for quickly finding files. These moves are largely considered to have been made by Microsoft to win over Windows 7 users who have not updated to Windows 8 due to unfamiliarity and a steeper learning curve.
For developers, there is application platform and one store for all the apps, which can run on all hardware, regardless of form-factor. These apps are called 'universal apps' because they can be run anywhere.
On March 30, 2016 during their annual Build conference, Microsoft announced a Windows 10 Anniversary Update that would come as a free upgrade to current users. The update includes new features thanks to feedback from Microsoft's Windows Insider program.
