June 26th will be the first time for the general public to, legitimately, experience the preview build of Windows 8.1, Windows Server 2012 R2, and the all blue'd-up Windows Server Essentials. This preview will be available through the Windows Store, but will not be upgradable to the final code.
Again, this preview will require, basically, Windows re-installation when final code hits.
Disclaimer aside, MSDN and TechNet subscribers got the updates ahead of time for server operating systems. Included with this release, a feature dubbed "Desktop Experience", providing the same interface as Windows 8.1.
Ars Technica played around with the updates and compiled several captioned screenshots into a carousel. Check them out. Of course, things could change, but this should be a better indication than — for instance — Windows 8 Developer Preview.
The final version of Windows 8.1 is expected to be released at some point later this year. The update will be free for current Windows 8 users.