Vocaloid is a computer software developed by the Yamaha Corporation, famous for its musical software, motorcycles, and pianos. Vocaloid is intended to be used for the purpose of creating songs, as it is a singing synthesizer. To use, one inputs a melody and lyrics into a pianoroll interface, selects the voice(s) they would like to sing the song (i.e. the famous Hatsune Miku), and selects the 'Play' function of the software. There are currently 25 released Vocaloids, which can sing in English and Japanese, and in a short matter of time, in Chinese, Korean, and Spanish. Vocaloid is usually either loved (by fans of J-pop and anime) or detested (by those who are not J-pop or anime fans). The most popular are Hatsune Miku and Kagamine Rin/Len. Users of Vocaloid to create original songs are commonly referred to as "Producers". Notable "Producers" are 'Akuno-P/mothy', whose complex stories bring a tear to one's eye, 'Owata-P', who makes viewers laugh with his silly songs, and 'Machigeriita-P', who has successfully given Vocaloid lovers the chills with his horror-filled masterpieces.
Honestly, Vocaloid is a truly fickle thing: one moment, you hear Megpoid: Gumi singing about time travel (Time-Warped By Chopping My Stag Beetle; I kid you not, that is the song's real name), and the next about committing double suicide with her boyfriend (Sunset Love Suicide)! Guess it depends on the producer, though.