One plugs into the same slot as your hardrives, and the other requires a SCSI card that plugs into the motherboard then cdrw into you scsi. The scsi setup is far superior to the IDE interface because you can achieve reliable burns, however there has been dramatic improvements with the IDE interface.
The scsi interface uses little or no cpu utilization where as IDE tend to use alot. With todays fast processors and and pc133 interfaces, IDE is has come a long way in performance and reliabilty making the IDE a better buy, where as you pay a premium for a scsi interface.