What is UTAU?
UTAU is a voice synthesis/virtual singer software. The user can select from a variety of available voicebanks, and then create songs by inputting the notes. UTAU will then make the voice bank sing.
UTAU is available for free here. Be warned that UTAU is in Japanese by default, and you will need a patch (here) to translate it. You may want to explore around the second link, which points to the UTAU wiki. They are a great resource for those just getting into UTAU for the first time.