I thought there was a pretty big difference between a high speed usb connection using a custom usb driver to interface a device with program on a pc and using hyperterminal to type messages to a robot.
using a USB to UART chip (such as the FT232 and the CP2102) means you don't have to write a driver. the chip manufacturer has done that for you and the chip will appear on your computer as a virtual serial port (COM port if you use windows).
you can then have any program running on your PC write text strings to the virtual serial port.
nice and easy.
writing USB drivers is somewhat complicated.
have a look here if you want to get involved in making USB devices without using a USB to UART chip: http://www.obdev.at/products/avrusb/index.html
(unfortunately the recent SOR web hosting problems wiped out the code from my own project there. i'll have it back up i the next few days.)
what's the big difference between a FT232 and the CP2102?
different manufacturer. read the datasheets for exact differences.