Windows 10 is not released that Microsoft is already working on its successor, or at least on its update. And its code name has already been found: it will be Windows Redstone. A nod to one of the minerals found in the game Minecraft, recently acquired by Microsoft.
Windows Redstone will therefore be in reality a big update of Windows 10, and not a partial overhaul of the OS as Windows 7 or Windows 8 could have been. This update is planned for 2016, and should be released in two stages: depending on the site Neowin.net, a first wave should be launched in June next year, and a second in October of the same year. Obviously, the content of this update remains completely unknown. But as Microsoft hinted a few months ago, Windows 10 should be the last change in OS numbering. Future updates to the operating system should simply be codenamed, and remain under the “Windows 10” flag.
Finally, remember that the finalized version of Windows 10 is expected by this summer. Microsoft is currently finalizing its OS and making all the necessary corrections. But the recent preversions disseminated by the publisher (the 10041, quickly followed by 10049) show that the main part of the new functionalities is present, and that there is not much missing on call.