The initiator device sends commands to targets to get them to perform a task. The target is responsible for receiving commands from initiators and acting upon them. Targets devices are typically SCSI disk drives, tape drives, CD-ROMs, scanners, etc.
Initiators are typically SCSI host adapters in computers.
Devices on a SCSI bus are assigned a unique identification number between zero and seven. Normally the host adapter is given an identification number of seven, but this is often configurable through software or jumper switches on the the SCSI host adapter. To send a command to a target device, a SCSI host adapter, acting as an initiator, selects a target device on the SCSI bus by asserting a number of control signals on the bus. The target device acknowledges the selection and begins to receive data from the initiator. Different pieces of information are transfered between initiator and target during what are called phases of the SCSI bus.