A hard disk needs some type of communications interface. In the case of SCSI, this is a SCSI interface. The commands are then interpreted by a disk controller inside the disk drive. There are no "simple" hard disks that do not have an interface or controller. You might be thinking about IDE hard disks that use the ATA interface.
Now it's a little more complicated than just saying the communications interface is different. The different interfaces are created by standards groups that also write functional specs. That means that some things that are done with SCSI (such as tagged command queuing) cannot be done with IDE disk drives. The controllers are also different.
This was first published in July 2001