SCSI can be as slow as 5MB/s or as fast as 160MB/s depending on the controllers, drives and cabling you use. The real advantage of SCSI is that it is much smarter than IDE. It allows for multiple drives to all be talking on the same bus in parallel.
IDE is limited to a single master and slave on a controller, and only one drive can talk at a time. If you have two hard drives as master and slave, it reads from one then stops reading and writes to the other. With SCSI though, you can have multiple devices on 1 bus, so you can have say 7 hard drives all on one chain. You can start a copy from drive 1 to drives 2, 3, 4 and 5, be working on a file on drive 1 save it to drive 6 and have drive 7 be your scratch disk all simultaneously. I wouldn't do that, but you can.
One of the significant reasons that you can do this is because each drive has a scsi controller, so it's able to intelligently handle traffic communication. Making the drives smart also makes the drives considerably more expensive. For $160 you can get a 9.1GB 7200rpm Ultra160 SCSI drive. Compare that to an IBM 40GB 7200 RPM 60GXP ATA/100 drive for the same amount.
The basic setup of a SCSI system would include a PCI scsi host adapter and a single scsi device with a cable running between them. Each scsi device typically has two connectors. To add another drive to the bus, you plug a second cable into the first drive and run it to the second drive. This is called a daisy-chain. There is no master/slave setting, but there are SCSI ID numbers and each device needs a unique number.
To make things complicated, there are many flavors of scsi, just as there is slow IDE up to Ultra ATA/100. In addition, different flavors of scsi can support different amounts of drives on the bus and have different limits on cable length. With SCSI 1, the speed is only 5MB/s and the bus can support eight devices. Eight devices doesn't mean eight drives, the scsi host adapter uses one, so that means seven other things can be connected. SCSI devices can be hard drives, CD-ROM, CDRW, Zip, Tape drives, Zip, heck even electron microscopes. Did I mention that the devices can be external as well as internal?
The fastest current SCSI runs at 160MB/s. To some extent it's possible to mix and match older scsi drives with newer drives, but doing so will typically cause the entire bus to operate at the slowest SCSI device's speed. So if you put a SCSI 1 scanner on the same chain as your Ultra160 devices, all the Ultra160 drives will run at SCSI 1 speed. (It is possible to run older devices on a segment of the bus that won't hinder the faster drives, but that's getting too deep.)
SCSI is most advantageous when high throughput is mission critical. Servers and video production are the most common examples. It's a serious step up from IDE and not for the faint of heart of the light of wallet. I know I've glossed over a lot of details and probably made a few honest errors along the way. This is all off the top of my head. The link above looks good, but it's a bit hard to follow the structure on the framed pages. Another good place to get an overview is
Adaptec's SCSI pages. They have some good, if marketing heavy, whitepapers on SCSI.
