UTAU© is a free, easy-to-use vocal synthesizer program created in 2008 by Ameya/Ayame for use by the general public. It has been used by millions, helping to produce vocals for original songs and covers alike, as well as bringing together a community of like-minded individuals to work together on projects and events (such as UTAUvision).
OpenUTAU is the spiritual successor to UTAU, and is an open-source vocal synthesis engine created by Stakira to address many issues users had with the previous program. It features multi-track editing and audio importing, allowing for easier editing for large projects.

A UST/USTx file, or UTAU Sequence Text File, is a set of instructions used by UTAU and OpenUTAU to make a VB "sing" using preconfigured samples. A UST contains all of the tuning and note configurations needed for a song, and can be edited at any time.