Hatsune Miku is a humanoid character from Yamaha’s Vocaloid software.
Her voice is created by samples from voice actress, Saki Fujita:
The name Hatsune Miku/初音ミク also has a meaning. Hatsu means first and Ne means sound. Miku is also a reading of mirai/未来 meaning future. So, it’s the futures first sounds.
Vocaloid itself is a programme you can use instead of a real vocalist. You can use the clips made to produce full vocals.
Here’s a live performance: