What is GPIO pins in Raspberry Pi ?

GPIO means General Purpose Input Output .This are the main and important part of raspberry pi.These pins are very helpful for developing Raspberry pi Projects.The pin outs are very different in each model the figure will given below for the reference.Each pin have different properties same functional pin have same color in above figure.These pins are used to connect the real world sensors to the pi.The pin yellow color is general purpose pins and others are special function pin also ground pins.