An icon is a way of providing information without the use of written text.
It shouldn’t be confused with a symbol which is a pictorial that doesn’t have to depict the object, or action that it is displayed for. An icon is intended to represent the object that is present.
For example, a speed camera sign on UK roads is a symbol. In the sign it uses a box browning camera, a device that is not the one in use at the side of the road, even though the early gatso camera’s did look a little like a giant yellow box camera. There is also no film in the camera. What adds further confusion is that the device in question is obsolete, very few drivers will understand what the pictorial is of.
A symbol has to be learnt, an icon however should be recognisable and familiar and representative of the thing. It’s also why I hate the funnel icon so much you often see being used to denote a filter. You do not use a funnel to filter a liquid, you use a funnel to transfer a large volume of liquid to a vessel of equal, or greater size with a shallow opening, or to ensure it is added in a controlled way.
A good example of an icon is the house often used on web applications and websites because it is denoting that you will be returned to the home-page. Although understanding the structure of a website and that every site has a home is a learned behaviour, it requires little cognition.
If you are looking at something and trying to determine whether it is a symbol or an icon – we can determine its purpose from what is drawn.