The short version is that Windows Insiders will get first dibs when Windows 10 launches on July 29, while everyone else will see a staggered roll out over the following days. In other words, if you haven't yet "reserved" your free upgrade from Windows 7 or Windows 8.1, now would be the time to do so. Microsoft began offering reservations through a pop-up message in the Windows notification center about a month ago.
Microsoft says it will use the rollout to check compatibility and to "listen, learn, and update the experience for all Windows 10 users." However, the company notes that "the vast majority" of PCs running Windows 7 and higher should be fully compatible. As a reminder, all users running Windows 7 or Windows 8.1 can get a free upgrade to Windows 10 within a year of the July 29 launch, after which the price will be $119.
As for retail, Microsoft will soon distribute a build of Windows 10 to PC makers, though it's unclear how many of them will have new Windows 10 machines ready on day one. Retailers will also be getting a Windows 10 build, so they can offer free upgrades to people who buy any remaining Windows 8.1 PCs. (Computers that are fully compatible with Windows 10 will have a sticker letting buyers know about the upgrade.)
For businesses, Windows 10 Pro will launch on July 29, alongside the consumer version, while volume licensing for enterprise and education customers will kick off on August 1.
Why this matters: New Windows versions and upgrades are usually a free-for-all, with Microsoft's servers struggling to keep up. This time, it seems Microsoft wants to avoid that situation by limiting the rate of downloads. Some users will likely be frustrated by this approach, but the good news is that Microsoft is experimenting with alternative methods such as peer-to-peer distribution for future Windows 10 updates.