Cortana is Microsoft's rumored Siri and Google Play competitor and is also rumored to be announced today at Build 2014 during the opening keynote. Cortana is a codename and it's never been clear if Microsoft will rename the technology or keep with the same designation of the public offering once released.
Many are hoping that Cortana will end up being the official name since it originated as an AI character in the popular Halo video games series. According to Bing today, Cortana may, indeed, be the official name. Yay.
In the lead up to the announcement, those that visit Bing and login with an account, can now go to the Personalization page in Bing Settings and get an early preview of Cortana management.