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You're only going to need to consider a SCSI hard drive if you're looking to build or update a busy, high-end server. SCSI (Small Computer System Interface). It is an older standard that allows multiple devices to be connected in daisy-chain fashion.
SCSI hard drives are high speed, but at the same time come at a high price and are not as easy to find as an SATA or IDE drive. They can be up to three times the price of a standard IDE drive.
Bottom line: If you don't know what a SCSI hard drive is, you probably aren't going to need one.