1) Here is a great article about microcontrollers and how they differ from a full computer: http://electronics.howstuffworks.com/microcontroller1.htm
tl;dr: A Raspberry Pi runs a full desktop OS (often Linux) and as such, can do most of the things a "typical" computer can do, such as browse the internet or install different programs. An Arduino, however, cannot do that. An Arduino can run one piece of software that you program to it (called a sketch in the Arduino world) and repeat that same program over and over.
2) They have different purposes. A Raspberry Pi is designed as a low-cost desktop computer that you can put anywhere, and also as a platform that is easy to put into different projects. An Arduino is better suited for attaching sensors to, controlling motors, etc.
3) see #2
Also, a Raspberry Pi is typically used when a GUI is wanted, or when more computing power is required. An Arduino is typically used for more real time things such as reading sensors or controlling LED's.
4) No, it does not. Arduino's run C++ compiled as machine code object files.
5) For the Raspberry Pi, Here is the list of official resellers: https://www.raspberrypi.org/products/model-b-plus/
For the arduino, it depends on what kind you're buying and where you live, there are many different styles.
6) As a general rule, Arduino's have more shields. But, a raspberry pi typically needs a screen, mouse, keyboard, etc. Everything a standard computer needs.
7) Entirely depends on what you're trying to do.
8) Both take a 5V input voltage. An Arduino can take up to a 20v input (again, depending on the model) whereas the Raspberry Pi can just be powered with 5v over USB.
9) Again, depends on what you're trying to do. A Raspberry Pi is "easier" to use, as it's a full computer.
