Cortana is Microsoft answer to voice recognition, it has been branded as a personal digital assistant when it was revealed at Build.
Cortana will also feature throughout Microsoft and the Windows platforms, including the new Microsoft Band fitness tracker. Along with building a feature rich product.
Currently Cortana can current be used to search for music (Similar to Shazam), roll dice or toss a coin, search the web, schedules, contacts and sport events. It is also integrated in a few apps, including Twitter, LinkedIn, Skype and even has its own notebook platform to store useful information about yourself for Cortana to learn from, which is the main difference between google now and Siri, the openness of information is definitely a plus.
Microsoft also have a few ways to integrate the speech functions into apps.