All Macs, except for the iMac (unless it has been changed) and laptops (notebooks), come with a SCSI port in the back of the computer. It looks a lot like the monitor port except wider and it has this symbol: 
. Every device should have 2 ports on the back: an "in" and an "out" (check your manual). The first device is hooked to the computer (from the deivces "out" port) with a wide to narrow SCSI system cable that should come with the product. All extra devices are hooked to the previous one from the "out" port of one to the "in" port of another with a SCSI peripheral interface cable; the last device in the chain needs to have a SCSI terminator on its "in" port. Also each device must be assigned a SCSI ID number (there should be a switch on the back). The numbers can be 0 through 6 (7 is used by the computer). If you have any internal SCSI drives (like a ZIP drive), it will be using a SCSI ID. Do not use a SCSI ID more than once. With SCSI you can only have up to 6 devices attached. Don't forget to install any drivers and have the device on before starting up the computer.
iMacs com with 2 built-in USB ports. They look like large telephone jacks. With USB, you just plug and play. If you want to have more than 2 devices attached, you need to buy a USB Hub. This hub has a cord in the back that plugs into a USB port. On the front are 2, 4, 6, or more USB ports that you can plug devices or more hubs into. It is possible for you computer to support up to 127 USB devices.
If you want to add USB to your PowerMac, you can. All you need to do is buy a USB Port PCI card and install it. I don't think you can do the same if you don't have a SCSI port.
PowerBook Users: You can use SCSI devices by buying a HDI-SCSI Adapter.