Hatsune Miku (Miku Hatsune outside Japan) is a Japanese vocaloid chatacter created by Yamaha. She's almost everywhere now in Japan. If You want to learn more about Miku, go to http://en.wikipedia.org/wiki/Hatsune_Miku .
Hatsune Miku (初音ミク in Japanese) translates to "the first sound of the future". This is so because when she was created, Miku was said to come from a futuristic world where all music is lost.