Cortana will be an installable app for iPhones and Android devices. While it won't have the same deep operating system hooks as the Windows Phone version--don't expect to conjure the virtual assistant with hardware buttons, for instance--it'll otherwise support all the same Internet queries and actions. For instance, users will be able to set reminders through Cortana on Windows 10, and get notified on iPhone or Android.
Further reading: Ask Cortana anything: Sassy answers to 59 burning questions
In Windows 10, Microsoft will also do a better job teaching users to sync their data from iPhones and Android devices. A new Phone Companion app will act as a sort of educational tool for existing Microsoft products, pointing users to the mobile app versions of Cortana, OneDrive, Skype, and Office.
For example, the app will teach users how to set up camera backups in OneDrive, so that smartphone photos will automatically save to the cloud and sync to the Photos app in Windows 10. Similarly, iOS and Android users will be able to stream any music stored in OneDrive through the Xbox Music app, starting as a beta feature in late June or July. (Windows and Windows Phone users already have this capability.)
The Phone Companion app will launch alongside Windows 10, which is scheduled to arrive this summer. Cortana will arrive on Android at the end of June, and on the iPhone "later this year."
Why this matters: Cortana's arrival on iOS and Android has always felt inevitable. Microsoft had made it an open secret months ago, and it fits squarely within the company's strategy of offering its online services across all platforms. The question is whether people will actually feel compelled to use Cortana--along with other services like OneDrive and Xbox Music--when there's no shortage of other options. All Microsoft can really do is make sure people are aware of what it has to offer, which may explain why it's announcing Cortana and its Phone Companion app in the same breath.