SCSI is the standard interface for workstation and server hard drives. While it is more expensive than IDE, this parallel interface can carry more bandwidth, supports more devices, runs over longer distances (up to 12m with popular forms of SCSI and possibly a little longer with older forms of differential SCSI), and multi-task.
A narrow SCSI bus carries eight addresses and a wide bus carries 16. The SCSI controller takes one address, which leaves another 15 addresses available for devices. The higher the SCSI address, the more priority the device has. This lets a system builder set drive priority but also adds a bit of complication to SCSI setup. Usually, its best to give time sensitive components more priority than bandwidth hogs like hard drives.
There are many variants of SCSI currently available, but we will just talk about the ones you are most likely to encounter in newer machines: Ultra, Ultra2, and Ultra160 SCSI. Ultra SCSI runs at 20MBps with eight addresses. There is a wide version of Ultra2 that uses twice as wide a data path to run at 40MBps. Ultra2 SCSI, also known as LVD (Low Voltage Differential) SCSI, runs at 40MBps. There is also a wide version that runs at 80MBps. Ultra160 SCSI continues the trend of doubling performance but only runs at a wide 160MBps.
SCSI devices are generally compatible with controllers and drives using other forms of SCSI, but there are no guarantees. Sometimes it is the case that, if you have an older drive on a newer bus, all the drives on that bus have to default to the speed of that slower drive. What actually happens depends on the controller you use. Also, using narrow cabling for a narrow device on a bus forces all other devices whose signal uses that cable to run at narrow speeds.