It is named after the artificial intelligence character in Microsoft's Halo video game series, Master Chief's holographic helper, Cortana.
Microsoft demonstrated Cortana for the first time at the Developer Conference Build 2015 in San Francisco. Albeit, Cortana is relatively new compared to Siri, which arrived in fall 2011 on board the iPhone 4S, it is seen as one of the key features in Microsoft's planned "makeover" of Windows Phone and Windows.
Comparison: Siri vs. Cortana
Apple's Siri and Cortana are both good at discerning natural language, however, Siri's many more years of evolution, and haven learned from the millions of iPhone users worldwide pitches her a little ahead of Cortana.
Cortana has an edge in understanding context, as it takes into account the last question asked, thereby performs remarkably well as with much of the functionality that Siri has acquired over the last few years.
Siri's integration with third-party app is a big advantage over Cortana at the moment. But, as already expected, Microsoft developers will eventually close that gap. Apple has promptly responded in its WWDC 2015, with iOS 9 update set to bring an array of new features, including proactive suggestions to Siri.
In conclusion, Siri vs. Cortana battle ground that has been drawn, will among other things ensure that mobile users get more out of their smartphone experience. And whoever wins, the users are the ultimate winners.