It's all right, I was confused when I first found out about them too. ^^ Well, VOCALOID is a software program. In the program, it uses a person's recorded voice samples to create a "singing" effect. (Fans make them too, those are called UTAU.) On the other hand, there are fandubbers who sing VOCALOID songs, such as Tolie, or Usa, or Yamai. People usually use VOCALOID to make original songs, or to sing covers of existing songs.
(I'm sorry if you already knew most of that. XD)
http://en.wikipedia.org/wiki/Vocaloid