Cortana is a female character from the Halo game series; however, she will take the form of a circular animated icon in Windows Phone. Similar to the Siri, Cortana will let call you by a nickname of your choice.
A key part of Cortana is the ability to save information and data in a Notebook system. Notebook will allow the Cortana digital assistant to access information such as location data, behaviors, personal information, reminders, and contact information. Cortana can also track flights or other mentions in emails, allowing it to generate notifications similar to Google Now.
Microsoft is expected to unveil Cortana at its Build Conference in June; however, it's unclear if the feature will still carry the same name when finally released.