I am trying to get a raspberry pi to issue commands(move forward, backward, turn left, turn right) to an arduino-based 
4-wheeled rover. 

According to this: 

https://raspberrypi.stackexchange.com/questions/1505/how-to-attach-an-arduino

if I use GNU Screen I can send commands over a USB port.  

So basically I will have a process running on the raspberry pi that determines which direction the rover needs to drive in.  This process will somehow "send" commands over the serial USB connection using "GNU Screen" as mentioned in the link above.  However, once the arduino receives the command(s), does it automatically start executing them? Or is there an additional step? 

Also, do I need to be concerned about the amount of power being drawn from the raspberry pi by the arduino? If so, how do I handle this? Would a USB hub suffice?

Would appreciate ALL/ANY advice or links to tutorials about how to get a raspberry pi and a arudino-based rover talking to each other.