Forgive me for not knowing anything, but...who exactly is Hatsune Miku...?

Well, if you're askin' for a specific answer, then she's a purchasable box set of hardware te can plug into your computer if you've got the right software to launch it. te can program her to make her sing along to a song. There are lots of others like her, and they are all called Vocaloids. Miku is the most popular, and video games are created that are rhythm based about her and the other Vocaloids.

She's a famous J-pop singer who is not really human. She's basically animated and has holograms and can sing in a much higher pitch than any human could

She is basically a hologram and a thing called a Vocaloid, they sing japanese songs at concerts all over the world for people who are huge fan of them like me! ^^. She is not the only Vocaloid there are others such as Len, RIn, Luka and Kaito te might have heard of them too. But I think she is the most popular. They also have games with them in it. Here is a picture of her in a concert.