Generally, one IDE channel will support two drives (HD, CD-ROM, etc), the master/slave is to differentiate between the two drives, so the BIOS knows which is which. Generally, if you have two drives, you want them on separate channels (IDE1, IDE2). When you have two drives one one IDE channel, you will take a slight performance hit, therefore, it is recommended that you put your hard drive as master on channel one, the Optical drives as master/slave on channel two.
