Ferdinand de Saussure

 The sign, the signifier, and the signified are concepts of the school of thought known as structuralism, founded  
by Ferdinand de Saussure, a Swiss linguist, during lectures he gave between 1907 and 1911 at the University of Geneva. His views revolutionized the study of language and inaugurated modern linguistics. The theory also profoundly influenced other disciplines, especially anthropology, sociology, and literary criticism. The central tenet of structuralism is that the phenomena of human life, whether language or media, are not intelligible except through their network of relationships, making the sign and the system (or structure) in which the sign is embedded primary concepts. As such, a sign -- for instance, a word -- gets its meaning only in relation to or in contrast with other signs in a system of signs.
In general, the signifier and the signified are the components of the sign, itself formed by the associative link between the signifier and signified. Even with these two components, however, signs can exist only in opposition to other signs. That is, signs are created by their value relationships with other signs. The contrasts that form between signs of the same nature in a network of relationships is how signs derive their meaning.

In Saussure's theory of linguistics, the signifier is the sound and the signified is the thought. The linguistic sign is neither conceptual nor phonic, neither thought nor sound. Rather, it is the whole of the link that unites sound and idea, signifier and signified. The properties of the sign are by nature abstract, not concrete. Saussure: "A sign is not a link between a thing and a name, but between a concept and a sound pattern"

• The linguistic sign unites, not a thing and a name, but a concept and a sound-image.
• The linguistic sign is arbitrary;
• The word symbol has been used to designate the linguistic sign, or more specifically, what is here called the signifier.
• The word arbitrary also calls for comment.
• The signifier, being auditory, is unfolded solely in time from which it gets the following characteristics: (a) it represents a span, and (b) the span is measurable in a single dimension; it is a line.
• Diachronic- a word over time
• Synchronic-any given moment
• Signifier(sound image) + Signified(concept) = Sign

 http://www.criticism.com/md/the_sign.html