The sole refers to the entire part of the shoe that sites below the wearers foot. It is usually constructed of several layers:
The insole is the part of the sole that sits directly beneath the wearers foot. Its purpose is to provide a comfortable layer above the joining of the upper to the sole.
A mid-sole can be found on some shoes, and is a layer between the in-sole and the out-sole.
The outsole is the layer of sole that is exposed to the ground. Due to the amount of wear and stress this part of the shoe receives it is usually made of a very durable material. It is also important that it provides enough friction with the floor to prevent the wearer from slipping.