USB Bridges are boxes that let you connect any two USB devices. Normally, USBs devices, like printers, cameras, and so are are meant to be connected to the computer. The computer is the hub and has the intelligence to deal with devices connected through USB ports.
Natually, this situation is not ideal for the photographer. We don't wan't to be taking along our computer on photography trips, but we might want to take along a printer and a backup storage device. Most cameras can print directly to printer, using the PictBridge interface. But how would you backup your camer's pictures on a mass storage device that wasn't designed to read memory cards? Like an iPod, for example.
With a USB Bridge. The cost is around US$80. Do a Google search for the term, or read about it at the Delkin site.