Microsoft Cortana which was first developed for Windows Phones has now reached Android and iOS platforms. Microsoft has developed Cortana very aggressively. Cortana has been launched for both Windows Phone as well as for Windows 10 on PC's.
Over the months, Microsoft has been beta-testing Cortana for both Android and iOS. Microsoft Cortana is akin to what Google Now is to Android and what Siri is to iOS devices. While Cortana functions differently from both Google Now and Siri, it has been designed for Windows platform. While Google Now is more search oriented, Cortana on the other hand is like Siri with a voice of a woman who greets users.
Windows users have loved Cortana and it has been a hit on Windows platforms. Apple and Google have both tried to adopt and steal aspects of Microsoft Cortana. In fact, Cortana has been a highlight for Windows phones if not a significant one.
Cortana for Android will allow users to use cortana in a more Window-ish way, if not like apple. Siri still rules when it comes to Apple. Cortana is unlikely to have freedom on the iOS platform owing to several restrictions of the iOS platform.Cortana for Android and iOS is unlikely to be like a replica of its Windows version. So people looking for a pure experience of Cortana or a more Windows like experience on their iOS or Android devices should probably go for a Windows Phone.
Cortana now also comes integrated with Cyanogen devices owing to Microsoft's partnership with Cyanogen. Cyanogen devices will have some more features of Cortana.
Cortana for iOS and Windows users is available only to users in the US and China. However, we can expect a global roll out probably next year, by the time Microsoft releases a Cortana that is more user friendly and accessible to users in Asia.