what is the difference between pi and Arduino?
Have you read the specifications? What don't you understand?
what is the difference between pi and Arduino?
Simplified: The Raspberry Pi is a complete Computer (PC) while the Arduino is a microcontroller. They have almost distinct fields of application.
The question is about the same as "What's the difference between a Piaggio (http://www.piaggiousa.com/scooters.html#!s=home/typhoon-50) and a Volvo VNL 780 (Welcome to Volvo Trucks)?" In both cases it's easier to tell what they have in common than what the difference is.