You're probably thinking of
Mini ITX (or possibly
nano or
pico).
I don't know much about these form factors except that they're even smaller than micro ATX. They're designed for very small, low power systems like you mind find embedded in cars, etc.
Very few of them use "desktop" processors. Most use something like the AMD
Geode processor. It's an Athlon-derivative that uses the x86 instruction set (32-bit). It uses considerably less power (and has significantly less processing power) than a desktop Athlon. This allows it to be passively cooled, thus saving space and noise.
Anyway, mini/nano/pico ITX is a specialty form-factor. Even assuming that you can find an ITX motherboard that supports desktop processors, I don't know why you'd want to. These boards don't offer expansion slots or anything that you would expect in a desktop system. You'd be stuck with whatever embedded GPU the board uses, which is definitely not going to be a 3D card of any sort. These things are mostly used as media servers, DVRs, etc., so they only need 2D capabilities. Finding a case, let alone a power supply for ITX factor is also going to be difficult since they really aren't so much a consumer product.
If you're going to build a desktop system, I highly recommend sticking to an ATX/EPS12V system, and a mid-tower case. Unless you have some really pressing need for a low-profile system, trying to cram everything into a tiny case using even Micro-ATX is just way more headache than it's worth. They're hard to work on, and you live in the perpetual shadow of thermal issues. If noise as a concern, you can still easily build a full-sized, fan-cooled desktop system that will run practically dead-silent.