Vocaloid Wiki

Vocaloid (ボーカロイド Bōkaroido) is a singing voice synthesizer. The software enables users to synthesize singing by typing in lyrics and melody. It uses synthesizing technology with specially recorded vocals of voice actors or singers. To create a song, the user must input the melody and lyrics. A piano roll type interface is used to input the melody and the lyrics can be entered on each note. The software can change the stress of the pronunciations, add effects such as vibrato, or change the dynamics and tone of the voice. Each Vocaloid is sold as "a singer in a box" designed to act as a replacement for an actual singer. 

The software became very popular in Japan upon the release of Crypton Future Media's Hatsune Miku Vocaloid 2 software and her success has led to the popularity of the Vocaloid software in general.