The FTDI FT245BM is a FIFO to USB converter with a parallel interface. The USB device emulates a serial port, which is COMx in Windows or /dev/ttyUSBx in Linux (http://www.ftdichip.com/FTProducts.htm#FT245BM).
With the parallel interface its easy to provide a high data rate connection between PC and AVR. The memory buffer inside the FT245xx for both, TX and RX direction, relaxes the CPU load on the AVR side. The MCU can write/read whole chunks of data to the chip without lots of interrupts or peramnently polling a USART status register.
The FT245 can be branded with custom vendor and product IDs in a external EEPROM. In combination with the hardware described here, the following combinations can occur:
The following sections describe, how the driver for different VID/PID in Linux and Windows is adapted: