You are probably wondering what a vocaloid even is. This question is very common and is often asked in places such as forums, youtube comments, and other such places. Most of the time these questions are taken the wrong way and die-hard fans get offended, leaving a noob without any answers. So I'm going to tell you now to avoid this certain situation.
Vocaloid is a voice synthysizing program that uses voice packs with names like Miku, Kaito, Luka..etc to create various songs and voices. The models like Miku became symbols for music videos and PVs which is where "Vocaloid" comes from. The models so far that have been created are Miku, Meiko, Kaito, Len & Rin, Luka, Megpoid, Lily and Gakupoid.
One popular video websites like Nico Nico Video or Youtube, people like to upload various music videos called PVs. You can see them on the Video part of this site. Vocaloid has gotten so popular that people made fanmade vocaloids like Kasane Teto and Akita Neru. Some fanmades were so popular that a new program similar to vocaloid was created, called UTAU.