SCSI drives are usually significantly more expensive than IDE drives. SCSI drives have a protocol that is used to communicate with the controller and operating system, while IDE drives are much simpler and can accept only a single request at a time. SCSI drives with tagged queueing can accept multiple requests and can reorder them to improve efficiency. This is why SCSI and IDE disks have similar performance characteristics for single-user or single-file I/O operations, but scsi performance is much better than IDE when the requests are generated by multiple users or processes. For this reason, SCSI is preferred for heavily-used database servers.
In fact, SCSI or IDE is only one way to generally distinguish between the two major types of drives -- enterprise drives, which are designed for high performance and reliability, and personal computer drives, which are designed to minimize cost. This paper, http://www.seagate.com/content/docs/pdf/whitepaper/D2c_More_than_Interface_ATA_vs_SCSI_042003.pdf, does an excellent job of describing the tradeoffs in producing drives based on performance/reliability or reduced cost. It is an excellent guide in choosing drives based on their features.