Universal Windows apps for PCs, phones and tablets: What are they?

Universal Windows apps for PCs, phones and tablets: 

What are they?

Microsoft at the Build 2014 conference announced Universal Windows apps, a move that brings a new common Windows runtime to PCs, phones, and tablets (and the Xbox One). Essentially, developers will be able to easily port their apps from one Windows platform to another, or create Universal Windows apps that with minor modification work across platforms.

All this is enabled by Microsoft through the common code and numerous shared APIs and UI controls between the Windows 8.1 Update and Windows Phone 8.1 platforms. Using Visual Studio 2013 Update 2, an upcoming release that developers got access to as a release candidate on Wednesday, app makers will be able to port their Windows apps to Universal Windows apps, or create Universal Windows apps that can be easily migrated to other platforms.