Microsoft will give those who tested the next version of Windows through the Windows Insider program a free upgrade path to Windows 10 when the OS is released to manufacturing, meaning you will be able to upgrade to the final RTM version without the need of a clean install.

Update: In light of the confusion caused by the ambiguity of some of Microsoft's announcements, you can find an article posted on  here explaining Windows 10's free upgrade offer.

When this news was first revealed by Microsoft's Gabriel Aul through Twitter yesterday, there was a bit of confusion as to how the upgrade would work, with some people speculating that the free upgrade meant a free Windows 10 license, however that is not the case.
Current Windows 7 and Windows 8/8.1 license holders will be able to upgrade to Windows 10 free of charge for the first year, meaning you do get a free upgrade. Those already using Windows 10 previews, will be able to upgrade to the final release code, but that won't make it a valid license outright.
Microsoft has yet to reveal how much Windows 10 will cost as standalone software. At the time of Windows 8's launch, Microsoft offered a short-term deal that allowed those upgrading from as far back as Windows XP to get Windows 8 for just $39. Of course, free is even better.
This is not the first time users get mixed signals from Redmond. Earlier this year Microsoft stated that pirates upgrading to Windows 10 will be able to do so without an existing valid license, but the license will remain invalid after the upgrade is complete. What seems clear is that Microsoft doesn't plan on giving away free licenses to anyone, but users with valid Windows 7 and Windows 8 copies will get a free upgrade for a limited time.
Windows 10 will launch this summer, potentially around the end of July, with a large array of new features. Those interested in testing out the OS ahead of release can still join Microsoft's Insider Program and download the latest preview build right now.