SCSI=Small Computer System Interface. While it's typically found on drives, it has been used on scanners and external CD-ROM drives in the past. However, with USB and Firewire/IEEE1394's PnP, SCSI has been dropped. Unlike IDE, SCSI can use up to 7 devices on a single chain. Also, they require a host adapter, which is more of just a doorway to the rest of the system rather than a controller of IDE.
Most motherboards do not have SCSI on them. Now, server and workstation boards may have them or have them as an option, but you won't find it on enthusiast or desktop motherboards.
SCSI is designed to be a robust workhorse, and typically carry a longer warranty than IDE drives. They start at 10,000RPM but can be found at 15,000RPM. SCSI drives today are 68-pin, but some drives made exclusively for hot-swapping maybe 80-pin.
If you want SCSI, then you'll have to purchase a PCI or PCIe SCSI host adapter. Depending on your needs, you can find them from around $50 to upwards of $500 or more, but these higher-end units have a separate processor for RAID calculations and onboard RAM and/or slot(s) to add RAM. Some may offer fibre channel, but that's really high-end stuff...and likely more than you'd spend on a computer.
SCSI is also a little more complex than IDE or SATA drives. You need to configure the drives by a device ID 0-7, but one of these will be reserved for the host adapter. Secondly, you'll need a terminator after the last device on the chain. Some SCSI cables may come with one, but if not you'll have to purchase one.
