Sign- An act or gesture used to convey an idea, a desire, information, or a command. facial expressions, speech disorders, slogans graffiti, commercials, medical symtoms, marketing, music, drawings, paintings, poetry, design. Signs can mean something other than themselves. 
index-Something that serves to guide, point out, or otherwise facilitate reference.

symbol-Something that represents something else by association, resemblance, or convention.