here is an easy an popular way to start using USB in your designs without going into learning the complicated USB protocol. This circuit converts normal USART signals from any microcontroller into USB compatible signals which can be directly connected to the PC. If u r designing a circuit and u need pc interface, then this is the best way, use USB, as the RS232 ports are disappearing from PCs and laptops very fast.
This circuit is as suggested by FTDI's datasheet for the device FT232BM. It also includes an EEPROM to input your device name which will appear in PC when u connect your circuit to PC. The Tx and Rx LED indication is also provided.
Drivers for this IC are available free on FTDI's website mentioned below. The drivers create virtual serial comm ports, hence keep the USB protocol completely in background. U can program yr controller just like u r designing it for RS232 communication!!
New! (21 May 2010)
FTDI has released a RS232 to USB converter in DB9 connector package, an easiest way to migrate from RS232 to USB without any change in your PCB, you can have the footprint of DB9 female connector but place this module instead and you are USB ready!! Really cool!!!
More info: http://www.ftdichip.com