What's the difference between Pi and Arduino?
We all know Raspberry Pi is a mini-computer running on Linux operating system; while Arduino is a microcontroller running on simple I/Os. Both of them focus on very different areas. Compared with Arduino, the Pi gets much better computing performance.
What happens when a Pi meets an Arduino?
A simple example would be, when building a mobile robot, we use the Raspberry Pi to extend its vision and get a nice tiny monitor. Meanwhile, the Arduino handles the motor driving and delivers fast responses like obstacle avoidance. The response time of rapid changes on the Arduino is much shorter without an OS. Also the high performance Pi can deals much easily with wireless communications, imaging and running complex algorithms.
Therefore, with the Arduino and Pi together, you get a powerful "brain" and "cerebellum" for your project, in a single solution.