USB Arduino-PC communication?

I'm a Junior Programmer (I have an Arduino Uno R3 board), but I'm not an expert in Electronics.

How is it possible to make the PC Operating System (Shell or Programming Language) control Arduino electric/electronic inputs/outputs?

Is it possible to do this using the USB port which is also used for programming the microcontroller?

Is there an easier way to do it?

For example, when an electric/electronic Arduino input changes its state (receiving or ceasing to receive electric current), the PC Operating System can intercept this event doing something (sending an email), or when a PC Operating System event occurs (receiving an email containing a particular string), it can change an Arduino electric/electronic output state (turning on a led): how?

Could you tell me where I can find information in this regard?

Thanks.

Have you done any research yourself?

http://arduino.cc/en/Reference/Serial

http://arduino.cc/playground/Main/InterfacingWithSoftware

This thread might help. Discussed here recently.

http://arduino.cc/forum/index.php/topic,101231.0.html