eeeh ok, let's start from the basic.
IDE is the common harddrive, what you are probably using. It is also call ATA or ATAPI. IDE drive have 40 pins (I think?). IDE device are slower and less reliable than SCSI. Plus it can't handle more than 4 device at a time. But IDE are cheaper ( $100 US or so for a 80GB, instead of $200 or so for a 36.4GB SCSI).
SCSI are mostly used for server, first, because they are faster (right now up to 20000 RPM compare to 7200 RPM to IDE). Also, a big problem with IDE is that only 1 device can be used at a time, mean that if you are writing on a certain hard drive, you can't use your second one in the same time (this is theory, pratically, today's IDE are faster than before and device are almost operating at the same time due to some algorithm using the system wasted time). So, SCSI are about 30% faster than IDE and this difference increase when you use more than 1 hard drive at a time.
Second, SCSI are more reliable than IDE. The ATBF (Average Time Before Failure) for IDE is about 2 years (this is an average remember) compare to a ATBF of 10 years for SCSI.
Third, it is a bit annoying, when you manage a "real" server, to be limited to 4 harddrive. Trust me, very big computer like mainframe need more than 4 harddrive
SCSI "chain" can handle up to 32 (or 64? not sure, actually) devices and you can add some controller cards to handle more. (Aren't you tired to be limited to 4 devices? I am!
)
So in conclusion, IDE are cheap harddrive for common users, cheap, fast enough and big enough. But real geeks or more powerful computer or server machines ask for SCSI. If you have money to waste, go for SCSI, else, stay with IDE.
