So what is this UTAU business, anyway?
If you are currently browsing this website, there is an overwhelming chance that you use UTAU already. However, in the miniscule chance that the freaky voodoo powers of Google brought you to this page knowing nothing about UTAU or even singing synthesizers in general, this page is for you! So, let's start from the top:
What is a singing synthesizer?
Exactly what it says on the tin. A singing synthesizer (like any voice synthesizer) uses voice samples, "resamples" them, and "reassembles" them to create a coherent singing voice. It's much like synthesizing any other instrument, but more technically difficult since you're dealing with all the different phonemes. The most popular commercial singing synthesizer is the VOCALOID software.
So what's UTAU then?
UTAU is a freeware singing synthesizer software with one particular feature that sets it apart from VOCALOID: anybody can create their own voicebank. Voice files are stored in .wav files, and configured using a .ini file. As a result, there are thousands of voicebanks ranging wildly in quality.
Great! So does that mean I can make my own?
Yep! There's a few things you should take care of before diving in, though. If you have Windows, the first step is to change your locale to Japanese. Mac users can use UTAU-Synth, but since I don't use a Mac I don't know a lot about that. I'd also personally recommend playing around with existing voicebanks first.
Why are most UTAU voicebanks Japanese?
We're all a bunch of weaboos
One reason is that the UTAU and VOCALOID fandoms are closely tied to the Japanese "doujin," or independent music scene. However, the largest reason is because Japanese is simply much easier to synthesize than many other languages.
Will you help me make my voicebank?
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Additional Resources
UTAForum - Discussion, resources, tutorials
Unaware's Guide to UTAU - UTAU guide I wrote in 2012. Be aware that it's slightly outdated.