You can connect two devices such as two hard drives, or a hard drive and CD/DVD drive, or two CD/DVD drives. There are two methods that can be used:
Cable Select - Jumpers on both devices are set to cable select. The device at the end of the cable is master and the one in the middle is slave. Cable select only works with an 80 wire cable (not the old 40 wire cable).
Master/Slave - One device is jumpered as master, and the other is jumpered as slave. In this configuration master and slave are determined by jumpers, not by position along the cable.