Windows phones are getting a personal assistant in the Windows Phone 8.1 update that will replace the built-in Bing search. Cortana is a little blue ring, not a full 3D blue woman, and it is Microsoft's answer to Apple's Siri and Google Now, the latter of which is news to me despite owning an Android phone.
Cortana has the direct conversational and question-answering abilities of Siri, but has broader capabilities like Google Now. It can track flights, save pertinent data in its Notebook system for future use, set notifications, and more.
Yes, you can have Cortana call you "Master Chief." Creep. Though Microsoft is calling its service "Cortana" now, in testing, it may not be the final name.
This is Cortana, Microsoft’s answer to Siri [The Verge]