Microsoft promised to bring its Cortana digital personal assistant to iOS and Android in May this year, and public beta for Android came out in July, with a private iOS beta following a few months later. Now both versions have been released, with Cortana available in the App Store and in the Google Play Store, at least for US users.
We looked at the Android preview when it first came out. The version shipping today isn't hugely different from what we saw in July. The Cortana app looks essentially the same whether on Windows 10, Windows 10 Mobile, iOS, and Android; search box at the bottom, hamburger menu on the top left, and information cards. Those cards can show appointments, track packages, give you weather information or the latest sports scores, and so on. Cortana requires a Microsoft Account to use, and will sync your various interests and settings between all Cortana-enabled devices.
There are some differences in capabilities due to the API availability of the respective platforms. Neither iOS nor Android supports opening apps or changing settings from within Cortana. On Android, "Hey Cortana" voice activation is supported on the Android home screen, but not system-wide. On iOS there's no hands-free voice activation at all. Missed call notifications are similarly Android- and Windows 10 Mobile-specific: miss a call on your Cortana phone, and your Cortana Windows 10 PC will tell you. You can write an SMS to reply to the caller on your PC, and through the power of Internet syncing, the SMS will get sent to the caller from your phone.