This shield board from DFRobot is designed to combine your Raspberry Pi computer board with an Arduino board.

Why combine a Raspberry Pi with an Arduino?
The combination of a high-performance computer board, and a microcontroller board with fast responses, offers a powerful core for your project. A Raspberry Pi and Arduino cluster opens up many more opportunities for your creativity and designs.
An example is in robotics. The Raspberry Pi board can control certain aspects wireless communications and complex algorithms. It can even provide functionality for a touchscreen interface. The Arduino then provides fast responses and obstacle avoidance.

More about the DFR0327 Shield:
Microcontroller: ATmega32u4
Arduino Leonardo Chip
Arduino Compatible pin mapping
Compatible with all Arduino standard shield and sensors
Arduino Digital I/O: 20
Arduino Analog I/O: 6
Raspberry Pi B+ Connectors: 16 x GPIO, 1 x I2C, 1 x ID_I2C, 1 x SPI, 1 x TTL UART
Voltage: 5V
Dimensions: 86mm x 61mm x 26mm (3.46" x 2.20" x 1.02")

Which Raspberry Pi boards is it compatible with?
Raspberry Pi Model B+
Raspberry Pi 2 Model B
Raspberry Pi 3 Model B
Raspberry Pi 3 Model B+

Supplied with:
Arduino Expansion Shield for Raspberry Pi B+ x1
20 cm micro USB cable x1
CR1220 button cell x1