Cortana had a beta leaked some months ago and it was possible to test it from there,but now its available to download from the play store direct,it is still a public beta though for now its for US ONLY but there is ways around that..It will however be more widely available soon and on iOS..
Cortana is your personal assistant and is now available on your Android phone so you can access her at any time.
Cortana is there to help make things easier for you and keep you up to date on the things that matter to you:
• Use Cortana as a voice assistant by speaking naturally or even by typing, and she’ll find answers from the web or help you get things done.
• Find answers using Bing including the latest sports scores, movie show times, local restaurants, facts and references, and more.
• Set reminders with Cortana and get the reminders at a specific or recurring time, or when you arrive/leave a certain location.
• Set alarms.
• Place calls to your contacts using voice.
• Voice texting: dictate your SMS/text to Cortana using voice.