Universal Serial Bus
Unlike other serial interface technologies such as RS232 that is free to use, USB is not. If you want to design a USB product then you have to pay a 1500$ fee each year for getting ID numbers to use with your USB device for operational. http://www.usb.org
USB got four wires.
+3.3v D+ D- GND
The USB hardware select between full speed usb2.0 and lowspeed usb1.0 by examine if any of the data wires D+ or D- got a pullup resistor connected.
1.5Kohm connected to D+ from 3.3V = fullspeed USB
1.5Kohm connected to D- from 3.3V = lowspeed USB
The USB-host is a little dictator that gives each device a slice of each juicy 1ms timeslot that is distributed to the USB-devices.
USB-devices can be powered from the USB cable, if the USB-device feel suicidial when connected then it can ask if the USB-host wants to send over some go-up-in-smoke frying current. (500 mA max)