Sign: Form of communication in things that are not always transparent to us. Composed of the signifier and signified and are sometimes arbitrary.
Ex: Woof = barking = dog.
Dog = Understandable to language = Four-legged animal
Index: The non-arbitrary part of a physical causal relationship between the signifier and the signified
Ex: Ticking is caused by a clock.
A black eye is caused by a punch.
Symbol: Any sign where there is an arbitrary relationship between the signifier and signified.
Ex: A black tie = Arbitrary relationship = A formal occasion.
Brrrr = Arbitrary relationship = It's cold outside.