The I2c(2 Wire Interface) and SPI(Serial Peripheral Interface) can be also used as GPIO pins when not being used in the BUS modes, and UART pins can also be used if you boot with serial console disable giving a total of 17 Input Output pins The RPI Revision 2 has an additional 4 GPIO (General Purpose Input/Output) lines on a different connector which we need to solder onto the board. The are 8 GPIO pins these and these ports can be programmed as either input or output, one of this pins can be assigned for PWM (Pulse with Modulation in addition there is a I2C( 2 Wire) interface and 4 wire SPI(Serial Peripheral Interface) with a second select line, making it five pins total, the serial UART(Universal Asynchronous Receiver/Transmitter) with a further 2 pins.

The Raspberry Pi has a 26 pin GPIO (General Purpose Input/Output) connector and this carries a set of signals and buses.

WiringPi is an Arduino wire like library written in C, In Arduino wiring is the core input and output for the Arduino, and wiring is already part of the Arduino IDE software.

Arduino wire library for raspberry pi