The software is available to all through the Developer Mode on Windows Settings and it is not a virtual machine. Microsoft will allow native ELF binaries, written for Linux, to run under Windows through a translation layer. It is a bit like the WINE project, which runs native Windows binaries on Linux.
Normally you have to recompile Linux software under Cygwin, or run a Linux virtual machine to get it to run in Windows.
Microsoft claims the new feature offers a considerable advantage in performance and storage space. It also includes the bulk of Ubuntu's packages, installed via the apt package manager directly from Canonical's own repositories.
The big question is why. Redmond does not appear to be targeting the server market with this launch but desktop and laptop users. It appears to be mainly of use to developers, who need access to Linux software but for whatever reason wish to keep Windows 10 as their main OS.
Canonical's Dustin Kirkland said the Windows Subsystem for Linux nearly has equivalent performance to running the software natively under Linux. The only downside is the software is free, but not open source.
General release is scheduled for later this year as part of the Windows 10 Anniversary Update, which will also include support for running Windows Universal Apps on the Xbox One, turning any Xbox One into a development system, the ability to disable V-sync for games installed through the Windows software storefront, ad-blocking support by default in Microsoft Edge, and improved stylus support.