It stands for 'Small Computer Systems Interface'. It's an alternative kind of device interface to IDE. It's not really all that much faster, but it's more consistent. Still, it is a bit faster. If you need to write data quickly (like if you're writing video to a file on-the-fly) then it's pretty essential, but for most purposes it's not worth it.

The easiest way to tell if you have any SCSI devices is to look at your computer's boot-up sequence. On the screen that shows up after the memory checks, there is a box that contains details of all your IDE devices. If you have an ATAPI CD-ROM drive (ie. an IDE drive) it will show up in that box along with any IDE hard drives or other IDE devices.

If you have a SCSI controller in your system, there will usually be another screen after the IDE screen that will display information about any SCSI devices you have on your system.

Your CD-ROM is almost certainly IDE, not SCSI. You will probably not see the SCSI screen since you probably don't have a SCSI controller.