The iMac (Flat Panel) has five Universal Serial Bus (USB) ports that are used for connecting the keyboard and mouse as well as additional I/O devices such as printers, scanners, and low-speed storage devices. Three USB ports are on the iMac itself; two more are on the keyboard.
The first USB port (the one nearest the mini-VGA connector) and the internal modem share one USB root hub; the second and third USB ports share a second USB hub. Each hub provides a 12 Mbps data transfer rate that is shared between the devices connected to it. See "USB Interface".
For more information about USB on Macintosh computers, please refer to Apple Computer's Mac OS USB DDK API Reference and the other sources listed in "USB Interface".