It's a pretty complex program that's basically allows users to manipulate voicebanks, creating whatever songs they like. And there's more than Japanese Vocaloids (one of my fave Vocaloid is Oliver, who has a British accent). English companies make them too. Crypton, the company that made a lot of popular Vocaloids like Miku, is very encouraging for aspiring artists and is pretty open to expanding the Vocaloid culture.
Of course, since it's really big in Japan, there's a lot of J-pop stuff out there. Though, there's some gems that I like. As for Miku, yeah, World Is Mine is very popular, but I'm not a big fan. There's better Miku songs out there imo.
Oh, if you're curious about all the technical stuff about the Vocaloid software like I was, check out this
article. Really goes in-depth about explaining the software.