How to read Raspberry Pi i/o pin diagram (GPIO pin graph)?
GPIO pin graph
Raspberry Pi i/p pins are located in the upper left corner of board, see following picture:

These pins are combination of Voltage supplies, Grounds and GPIO (general purpose input/output) pins. You can distinguish them from following graph:

Understand those pins functions are fundamental to design projects which allow Raspberry Pi to communicate with sensors and many other devices.

● GPIO are your standard pins that simply be used to turn devices on and off. For example, a LED.
● I2C (Inter-Integrated Circuit) pins allow you to connect and talk to hardware modules that support this protocol (I2C Protocol). This will typically take up 2 pins.
● SPI (Serial Peripheral Interface Bus) pins can be used to connect and talk to SPI devices. Pretty much the same as I2C but makes use of a different protocol.
● UART (Universal asynchronous receiver/transmitter) are the serial pins used to communicate with other devices.
● DNC stands for do not connect, this is pretty self-explanatory.
The power pins pull power directly from the Raspberry Pi.
● GND are the pins you use to ground your devices. It doesn’t matter which pin you use as they are all connected to the same line.