I assume yo uare talking aboue the IDE/AT interface.
Master: Typically it is the boot disk, or the first disk on the IDE bus. Also known as drive 0.
Slave: Typically the second drive no matter what bus and is commonly referred to as drive 1.
You cannot have two masters, or two slaves connected to the same cable of the IDE bus. If your motherboard has two IDE connectors (and most do).
The primary IDE bus is normally used for the master boot drive.
While you can mix and match what types of IDE devices are connected to each bus. It is a good idea to connect your hard drives to one bus (a master and a slave), while using the second IDE connector for CD-ROM and Burner drives, as the IDE bus will default to using the slowest data rate device connected to it as the data rate. (i.e. if you connect two devices and one runs at 10Mhz, and the other can run at 100Mhz, the bus will run at 10Mhz for both, substantially slowing the faster device).
