SCSI is pronounced "skuzzy". Now that is out of the way, here is the technical explanation. Every computer needs to be connected with peripheral devices once in a while. Basically, peripheral devices are those devices that are connected externally to the computer. Now where does SCSI fit in over here? SCSI is set of standards that are used to communicate with the peripheral devices. These standards are also used while transferring the data between computer and external devices. SCSI is generally used for communicating with hard drives. It is also used for communicating with other peripheral devices such as USB flash drives, CD ROM, printers and many data storage devices.
Consider a situation. There are three desktop computer hosts which need to use five peripheral devices. Now how will these hosts communicate with peripherals? Here a need of common platform arises. SCSI interface has been developed to provide such a platform. An important obvious thing I feel the need to mention over here is that, there has to be at least one host for any number of peripherals. SCSI interface is mainly used for hiding the complexity of the underlying physical format.
Integrated Drive Electronics(IDE) has a history of gradual development. After going through many technological advancements, it has evolved into what is today known as PATA. The underlying standard for IDE is AT attachment and hence the name PATA. The name PATA has been the retronym for IDE since the introduction SATA. In IDE standard, the cable length is limited to 460mm. Because of this, most of the devices that use IDE are internal to the computer. Before the advent of SATA, IDE was the most inexpensive and widely available standard in market. But SATA , in recent times has gradually captured IDE market.
If you think SATA is any different than SCSI or IDE, well their end result in functionality is pretty much the same. Just as SCSI is different from IDE, SATA also has some of its own characteristics. SATA is an acronym for Serial AT attachment. The work on SATA began with an aim of replacing the older IDE technology. The developers of SATA interface were successful in using the same commands that were used for IDE. As explained above, IDE is a parallel technology, while SATA is a serial technology. SATA also has an IDE emulation mode, which means that SATA interface can operate as IDE, if the computer motherboard is of old technology and does not support advanced host control interface. Basically its the PATA mode.
SCSI vs SATA vs IDE: Reliability
In the context of comparison, SATA is significantly more reliable than IDE. If the comparison has to made between SCSI and IDE again SCSI is far more reliable than IDE. SATA has already left IDE behind, now it is gaining major grounds against SCSI.
SCSI vs SATA vs IDE: Performance
Continuous research on all three interfaces has increased their performance, but SATA has shown to be the most promising of all. SATA has greatly improved its performance in recent years, but SCSI has not been able to match such increase. SCSI is still out performing SATA, but it had high performance to begin with. SCSI still remains on top of the three interfaces, but the coming versions of SATA should out perform SCSI.
I hope that the above article on SCSI vs SATA vs IDE has shed light on their key differences. But in my opinion SATA interface would be the leading industry standard in years to come.