Microsoft will release Windows 10 to the general public on July 29th 2015. This is when existing users of Windows 7 and Windows 8.1 will be able to upgrade for free, when new PCs with Windows 10 will ship, and when you will be able to buy Windows 10 in-store.
There are an expected 350 Million users of Windows 7 and Windows 8.1 that will be freely upgrading to Windows 10 by the end of the year. This could cause major slow-downs for Windows update. To help with that Microsoft has started a reservation system. Current Windows 7 and Windows 8.1 users have come across a Windows logo in their system-tray. This launches an app that lets you reserve your upgrade.
When it is your turn in the reservation line Windows will download the Windows 10 upgrade image from Microsoft Update. You will have the option to do a clean install, or upgrade a current installation.