Windows 10, by default, has permission to report a huge amount of data back to Microsoft. By clicking through “Express Settings” during installation, you allow Windows 10 to gather up your contacts, calendar details, text and touch input, location data, and a whole lot more, and then send it all back to Microsoft so that it can be used for personalisation and targeted ads.
This isn’t entirely unusual: recent versions of Windows, unless you explicitly say otherwise, have reported some kind of data back to Microsoft. Windows 10 definitely goes one step (well, a few steps) further though, primarily thanks to Cortana (which ideally needs to be personalised/optimised based on your voice inputs, calendar, contacts, etc.), and other “cloudy” features that somewhat necessitate the collection and squirting of personal data back to Microsoft.
That isn’t to say you should be happy about this state of affairs, however. If you’d like to retain most of your privacy and keep your personal data on your PC, Windows 10 can be configured in that way. Just be warned that there are quite a few toggles that need to be turned off, and you’ll lose some functionality as well (Cortana won’t work, for example).