Historically, Macintosh models have supported a number of interfaces for peripherals: SCSI (Small Computer System Interface), ADB (Apple Desktop Bus), and serial ports. The iMac has none of these. Instead, it supports USB (Universal Serial Bus).
USB holds several advantages over these other interfaces:
Hot-pluggable. This means you can plug in or disconnect devices while everything is turned on. Do this with a SCSI device, and you're likely to fry the motherboard; do it with an ADB device, and you could blow a fuse.
Simple driver installation. It's a drag-and-drop or installer thing. Drivers for many devices come preinstalled with Mac OS on iMac computers.
Easy connections. You don't have to deal with terminators, memory addresses, or ID numbers. There's only one kind of cable, soyou're never stuck with the "wrong" cable.
Greater expandability. USB supports the simultaneous connection of up to 127 devices, by attaching peripherals through interconnected external hubs.
Better performance. USB is much faster than traditional Macintosh serial and ADB ports.
Although it's true that you cannot simply connect the SCSI, ADB, and serial peripherals you may already own to an iMac, several companies make interface cables that can bridge the gap. And, of course, there are many new USB peripherals available or in development. You can check Apple's USB Web page for the most up-to-date details.