I am confused with hard disk nomenclature. Are Ultra DMA 100 and Ultra ATA 100 the same thing? Or is it just another way of saying the same thing? Does a hard disk have to be special to conform to either or both of these? Is it just a type of controller on the motherboard? What exactly are these terms and their differences. Thanks for your time.

Check out this at PCGuide. http://www.pcguide.com/ref/hdd/if/ide/modes_UDMA.htm

ATA (ATAPI) is the designation for the overall set of standards or protocols for drives. UDMA is merely one component of those standards. The terms, however, tend to get used interchangeably in ads and other places by folks who don't seem to know or care that there is a difference between them. So if you see ATA-100, know that UDMA-100 is what is (in all probability) really meant.

Only as add to RAC who explained it easy enough: DMA100 or UDMA100 or ATA100 means the drive con do up to 100MB/sec. To make full use of the drive you need the fitting ATA100 or higher controller on the board (or controller card) but those standards are all downward compatible. I'm running ATA33 drives on my ATA100 controllers and a ATA66 drive on the ATA33 controller of my old Asus P2B board. This means that controller runs at the lower speed so you shouldn't mix a fast and a slow drive on a fast controller if you can avoid it. The only problem that can happen is with additional build in raid controllers and very old drives. I a couple of pre DMA (ATA33) drives (130-515MB) who still use PIO Modes and the additional raid controller on my board can't detect them anymore!