Hatsune Miku is a “vocaloid”. A vocaloid is the banner term for a Yamaha singing voice synthesizer. Think of one of those funky text-to-speech functions but with the additional option to put notes into it like a keyboard. Like Hatsune Miku, there are avatars and personas for the other vocaloids. There are even a bunch of fan-made vocaloids with their own personalities made by fans.