From Vocaloidism Wiki
Vocaloid is a singing synthesis application software. The term Vocaloid was originally the name assigned to the Vocaloid engine, but has come to encompass the engine, voice package, and character associated with the voice as a single entity.
 Vocaloid Engine
The Vocaloid engine is a software developed by Yamaha Corporation. The engine is designed to synthesis singing vocals from notes and lyrics given by the end user. There have been two versions of the Vocaloid engine, the original Vocaloid engine and the newer Vocaloid2.
The Vocaloid engine is only able to synthesise voices into singing, and does not include the voices required to create singing. Yamaha sold rights to other smaller companies to record various voice packages for the engine. Voice packages recorded for the Vocaloid engine cannot be ported to the newer Vocaloid2 engine and voice versa, as the method of recording for each engine is vastly different.
 Voice Package
Voice packages create different vocals depending on the voice of the person the voices are recorded off. The resulting voice package is a Vocaloid singer. The voice package, bundled with a copy of the Vocaloid engine, is then marketed together.
Since the Character Vocal Series, fans have created personas for each Vocaloid singer. The images associated with the Japanese Vocaloids are official, and promoted by the companies who released the voices. The English characters do not have official personas since they were not marketed as part of the Character Vocal Series. But there are fan representations of them on the internet.
The available Japanese vocaloids are:
The English vocaloids are: