UTAU is the free/more open/open source version of VOCALOID.

VOCALOID is a software by Yamaha/Crypton that synthesis singing vocals. Roughly, you put in the notes, and what should be sung where, and you got the computer singing! :D

To use the software, you need a voicebank, which is often also called a/the VOCALOID. The most popular and widely known is Hatsune Miku! (see drawing below!). These voicebanks often have an accompanying character with artwork, personality, etc. ...

UTAU offers a nice starting point, being free and it's most accessible version, OpenUTAU, being open-source. You can use user-made voicebanks, or even create your own! Which is something I did after becoming more familiar with the entire topic.

UTAU offers you the ability to tune the voice to your liking, making it sound more (or less, if you want that) real; virtual voices definitely have their charm, signaled by their large following.