Vocaloids are characters that were originally created for the Vocaloid singing synthesizer software package produced by the Yamaha Corporation. The software began life as a research project spearheaded by Hideki Kenmochi of Pompeu University and later Voctro Labs. The software is capable of generating synthesized singing by allowing the user to input both lyrics and an accompanying melody. Initial commercial voice banks for the synthesizer were designed specifically with Japanese in mind, but support for other languages has become more prevalent.
The Vocaloid software package includes a singer library; essentially a collection of different synthesizer voice samples that can be chosen to sing the lyrics the user inputs. These voices are based on samples provided by actual singers. Characters for the various voices were subsequently created to represent Vocaloid for marketing purposes, and have since become popular in their own right. Hatsune Miku, the second voice created for the Vocaloid 2 software release and based on the voice of voice actress Saki Fujita, has become particularly popular, appearing in marketing for not just Vocaloid, but for products ranging from Domino's Pizza to automobiles.
Among her various crossover appearances, Miku began showing up in video games starting with titles such as Albatross18. However, she received her own rhythm game in Hatsune Miku: Project Diva, which contains a variety of songs composed using the Vocaloid software. Subsequent entries in the franchise included other Vocaloids such as Megurine Luka (notably, the first Vocaloid designed to sing in both Japanese and English), Kagamine Len and Rin, MEIKO, KAITO, and others.