It's a free upgrade for the first year. You will have until next July to upgrade your PC and, if you miss that Window, it will be a separate purchase.
New PCs will pay Microsoft for a license, same as always.
Microsoft will provide updates "for the supported lifetime of your device." They haven't really explained that phrase but my best guess means that, if your PC dies, don't expect to be able to transfer your Win10 license over to its replacement.
MS will expand the Windows Store to include desktop versions, digital music and videos (like iTunes) and will take a cut from in-app purchases made from Windows Store apps.
Win10 will come with shortcuts to quickly get you signed up for an Office 365 subscription and Skype (you need Skype credits to call people's phones)
This is all a big bold marketing move on Microsoft's part but it will get their latest operating system on to a huge share of the PCs out there.