Microsoft has slowly begun to roll out Cortana among iOS users.
With Cortana iS is the first mobile platform on which all three major language assistants are active. Apple has built itself Siri in iOS while Google has built into its app for iOS voice assistant, Google Now. Incidentally the hole with Siri are difficult to fill for the two rivals Apple because not every part of iOS access to third parties.
With Cortana iOS for some basic tasks can be performed otherwise fine. So you can send a message with your voice, appointments checking in and perform a search on Bing.