We know a rock is a rock because it is not a cat, a person, or a leaf. You can carry this out as far you would like. The purpose for naming things and defining them is to establish what they are not, thus what they are. A sign then is something which represents that which it is not. I'll make a few comments about each.
Realistic photographs look like the person, space, or object depicted.
This association is arbitrary and may hold no relationship to what it represents. The correspondence between the object of representation and what it represents are both historically and culturally determined. The meaning of an index is limited to one's interpretation within a culture or social group. A symbol, like an icon and index, refers to something which it is not. In this sense we can say, that object is symbolic.