Not easy to know everything about USB. The only person I know about that gets close to that is Tsuneo Chinzei which you find posting help about USB in a large number of forums all over the net.

The nice thing about USB is that the large USB protocol set includes virtual serial ports.

The image Andy posted shows a special data cable containing a chip that implements such a USB-to-serial interface. It is similar to ready-to-buy USB-to-RS232 interfaces available in every data store but with the difference that the UART side is logic level (3.3V or 5V variants exists) instead of RS232-level (where the data lines switches between positive and negative voltages relative ground).

Your microcontroller will think it talks with another UART.
Your PC will think you have installed an external USB-to-RS232 adapter.

Because of the framing speed of 1ms used by USB, the lag (response times to handshake changes or turned data direction) will be higher than having a "real" serial port, but the advantage is that a microcontroller (or some already existing hardware) that doesn't have a built-in USB device controller can be interfaced directly to the USB connector of a PC. And you can interface your equipment to a laptop that doesn't have any serial port, and no space for adding a PCI or PCI-E board with a couple of serial ports.

The special thing about the new cables Andy mentions is that the cables now also exists in a version that will negotiate the 5V power, asking the PC to be allowed to use the full 500mA that is the limit for all USB ports before the new 3.0 standard where the maximum current has been increased to 900mA.