use Wikipedia for in-depth details but basically
Itx is a form factor like Atx it comes in two sizes Mini-itx and Micro-itx. The larger and more adaptable Mini-itx has a motherboard measuring 6.7 inches and is square in shape. there are only 3 manufacturers of mini-itx motherboards Via (the people who built the original motherboards which consist of integrated cpu chips which aren't very powerful), Commel (they create motherboards that support mainstream cpu's currently the most powerful cpu you can place in one is the C2D (mobile) which are the intel T7600,T7400,T7200), Can't remember the name (they make boards for servers and retail electronics such as dvd players). The cases for mini-itx are built for small form factor and generally only have space for one 3.5" drive and a slim-line dvd drive, they dont use standard power supplies but instead use laptop style external adapters there are a few exceptions though with internal power adapters but this often creates heat in the case.
You could use a server rack mount case and then just create a single rack on the back of your monitor through attaching two rails or you could build a custom case.
this my Itx build I am currently saving for:
motherboard: COMMELL LV-677 Intel Core Duo/Core Duo 2 Mini-ITX express Motherboard
Ram: G-skill 2x1gb
Hdd: 750gb seagate cuda 7200.10
case: Psile | Silent Media center or living room PC according to Nexus
extras: mini pci wireless card, Hauppage tv tuner
os: windows xp mce maybe vista
cost without tv tuner is just under £800 the best mac mini is £910 has a 160gb hdd and a core duo (mobile cpu)