Primarily these devices are portable hard drives (battery powered) that can read various flash media and transfer their contents to itself. So if you have limited amount of flash memory for your digital camera, you can still shoot gigabytes of pictures while away on vacation or even just out for the day if you're a shooting demon.
Secondary, some of these units also play back MP3 and/or other media, including the ability to view the images on a TV. They'll usually have some type of directory functions to sort files and delete files. They should also have some type of AC power ability.
Belkin also makes a device I long ago invented* (yes, I thought of it first). An attachment for the iPod which allows you to read CF cards and transfer their contents to the iPod's own HD.
Bruno
*I just obviously don't like making money or I would have constructed a prototype and sold the idea and design.