I've been using Cortana for a while on my Lumia 930 Windows Phone and it's pretty cool to have my own personal digital assistant on my phone. Later this year we will bring Cortana to PC's and tablets running Windows 10 and if you are on the Windows Insiders program you have most likely checked this out already. Over at the Windows team blog there is a post that goes into some detail about the history of Cortana as well a video from Marcus Ash about how Cortana comes to life on Windows 10. I've embedded the video below for easy reference and if you want to read the post for more details make sure you check it out here.
Looking forward to how this pans out throughout the year!