VOCALOID
To put it simply. VOCALOID is a program that utilises vowel and consonant sounds sampled from the voices of real life people to make music.
VOCALOID uses these 'sounds', also known as voicebanks, to create music. The system uses a piano roll type software to allow the user to input the sounds. VOCALOID, while having characters with avatars, is less of a singing software than an instrument.
Voicebanks on their own are not as impressive as they seem but the software also comes with the ability to change the voice tone and vocal effects. Some of them include 'growling' and 'breathing'. These parameters can drastically change the voice sound. This is most commonly known as tuning and every VOCALOID Producer has a different style.
note: these are not the only ways to tune and only exist for v4 voicebanks and up. there are other ways to tune, specifically with pitch bends. i don't know why i wrote these to be the only ways? but it is how it is.
Another parameter, though uncommonly used, is the 'gender' filter, which allows you to deepen or raise the tone of a voice to be more 'masculine' or 'feminine' (for want of a better term).
The program SYNTHV is very very well known for making 'humanoid' sounding voices. VOCALOID producer Mitchie-M utilises VOCALOID's engine and capabilities to replicate human voices, an example of which could be KAITO's tuning in Ohedo Julia Night and KAITO's announcer voice in Seraphim On The Ring.
*the vocaloid 4 editor.
You keep mentioning producers, but what are they?
You know how musicians are the ones who play the instruments in songs? Well producers are the ones who make the lyrics, music and tune the VOCALOID. Sometimes, multiple producers will work together, or different producers will create the backing track to the one who tuned them.
In the case of GIGA and Hachioji-P, the song GIMME X GIMME was a collaboration between producers. This isn't uncommon and happens a lot.
Another popular producer style is the Unnamed Producers. These include Azari and x0o0x_, who upload songs without titles. These songs are quite well known and not much is known about the producers. These videos typically can't be searched, although most of Azari's songs--like Shadow Shadow--do have titles that you can find on Youtube.
In the cases of SAMFREE (luka luka night fever) and WOWAKA (rolling girl, unknown mother goose, world's end dancehall), these producers are sadly, passed.
Most producers add the suffix -P to the end of their name, which signals the fact that they're a producer. Some examples are Pinocchio-P, Kurousa-P and Daitensai-P