"What's the difference between a SCSI drive and an IDE drive?" --- (shortened and imprecise version)
Small Computer Systems Interface is a protocol for moving data on a path between different devices on a bus between computer motherboard and hard drive(s) and various other drives and/or printer, scanner, &c. It is - or was fifteen or twenty years ago - a faster, better way to move data than the Advanced Technology Attachment / Integrated Device Electronics protocol common to Intel machines. It came with a controller on the motherboard that served as a traffic cop to send all the data in the proper direction and to the proper destination(s). It also required a special woven ribbon that differentiated between the numbered (jumpered) devices, reduced 'crosstalk', and terminated the path - sending the data back in the other direction. In the advance of progress, ATA improved much faster and especially much more cheaply - particularly with the help of the internet - as a way to move and store data. Later Macs adopted the ATA standard, and when they came with a SCSI system they included a separate controller card to replace the previously built-in controller.
Your ACARD or Sonnet controller card allows your hard drive to behave like a SCSI system.
A practical difference is that for an 80GB HD, ATA is about $200 less expensive.
Hope that makes it all perfectly clear!
Regards... jws