sign: Something that suggests something not immediately apparent
Verbal example: "you are a very nice guy and all..." a common statement saying i am just not into you and i see you only as a friend
Visual example: a sign with a bicycle on it suggest to drivers to be weary of bicyclers
index: Something that directly points out something
Verbal example: "watch out" an exclamation to make sure everyone is alert.
Visual example: the index at the back of a book can give the exact place for information
symbol: Something that represents something else
Verbal example: many analogies and metaphors represt another meaning
Visual example: swastika was originally a symbol for many eastern religions but was later adopted by the nazi party durning hitler's rise to power.