Semiology, or semiotics, is the science that studies the life of signs within a society. It was born when French linguist Ferdinand de Saussure formulated his theory of language, published posthumously at the beginning of the century. Semiotics is not limited to language however – far from it. Everything that involves communication, even non-deliberate, is something that semiotics can tackle. This science has been applied to animal behavior, social habits, proxemics, architecture, poetry, mythology, and so on ad infinitum.

The basic definitions of semiotic elements follow below. Please note that I am not using the terms "symbol" and "sign" the way Saussure did: his original use had "sign" as a category under "symbol", but later on the two words were inverted so that "symbol" became the category under "sign". In order no to change terminology in mid-article, I'm using the more common terminology throughout.

SIGN (symbol in saussurian terminology)
Signs are events or things that direct attention or are indicative of other events or things. Basically, anything that represents something else is a sign. All the other definitions I give here are categories and subcategories of signs.
A sign has a certain structure that Saussure first defined as the association between a signifier and a signified. Let's take the letter A and say it represents an ox. The signified is the ox, what the letter A signifies. The signifier is the letter A. The sign requires the presence of both A and the ox. It is not, as is sometimes believed, the letter A alone, because if it has no signified -- if it has nothing to represent -- then the A is nothing but itself. It is not a sign, it has nothing to say about something other than itself. It remains a signifier, but one that is out of work.

There are three large goups of signs: the Icon, the Index and the Symbol.

ICON
Simply put, an icon looks like its signified. We are all familiar with computer icons, that helped popularize the word, as well as with the pictographs such as are used on "pedestrian crossing" signs. There is no real connection between an object and an icon of it other than the likeness, so the mind is required to see the similarity and associate the two itself.
A characteristic of the icon is that by observing it, we can derive information about its signified. For instance, if I don't know what a wolverine looks like, seeing an image of one will teach me a great deal about its appearance. The more simplified the image, the less I'll learn, but I will still learn. No other kind of sign gives that kind of information.

INDEX
An index has a causal and/or sequential relationship to its signified. A key to understanding indices (or indexes) is the verb "indicate", of which "index" is a substantive. Indices are directly perceivable events that can act as a reference to events that are not directly perceivable, or in other words they are something visible that indicates something out of sight. You may not see a fire, but you do see the smoke and that indicates to you that a fire is burning. Similarly, you cannot see sadness, but you can see the tears that indicate it. The word "this", like a pointed finger, are also indices. The nature of the index has nothing to do with that of the signified, but the connection here is logical and organic -- the two elements are inseparable -- and there is little or no participation of the mind.
Indices are generally non-deliberate, although arrows are just one example of deliberate ones.

SYMBOL (sign in saussurian terminology)
A symbol represents something in a completely arbitrary relationship. The connection between signifier and signified depends entirely on the observer, or more exactly, what the observer was taught. Symbols are subjective, dictated either by social convention or by habit. Words are a prime example of symbols. Whether as a group of sounds or a group of characters, they are only linked to their signified because we decide they are – and because the connection is neither physical nor logical, words change meaning or objects change names as time goes by. Here it all happens in the mind and depends on it.
Symbols are ideas, and whenever we use one we are only pointing to the idea behind that symbol. Do you know how computer aliases (or shortcuts) work? You create a file that opens the actual file it refers to. If you trash the alias/shortcut, it doesn't affect the file. Symbols work in exactly the same way in relation to the concept they serve. The $ symbol, astrological symbols, road signs, V of victory, are all symbols.
A symbol can rarely tell us anything more about its signifier than we already know. To return to our wolverine example, seeing the word "wolverine" on this page will teach me noting about it these 9 letter don't even say whether it's an animal or a rehab center for aggressive dogs. I am reminded of a text by a French author that I read in school: as a child, she would sometimes hear a word she doesn't know, and let her young mind imagine what it may be. Thus she decided that a presbytery was the shell on a snail's back, and was quite dismayed when she learned its real meaning.

At this point you can stop reading and you'll have a good idea of the way semiotics look at things. What comes next goes into much painstaking detail that only Virgoes like me can delight in.

Charles Sanders Peirce, the American philosopher, went further into the analysis of the Saussurian theory. His study does not diverge from Saussure's, except for two details:
- Peirce inverses the words "sign" and "symbol", making "sign" the general word and "symbol" the convention-based sign. It is this use that I have adopted throughout this article.
- Instead of the binary relationship of signifier and signified established by Saussure, Peirce uses a triangular model: object-sign-interpretant. To Peirce, a sign is anything that stands for something in somebody's mind. This "something" is called the sign's object; the "somebody" is called its interpretant. Saussure had collapsed the object and interpretant into one signified, a model that denies any possible difference between an object and our perception of it. Opinions still diverge as to which of the two had the best idea.
These 3 elements (object, sign, interpretant) form a triangle that is held together by a fourth: it is the ground on which the sign stands for the object (icon, index or metaphor).

The exact process of signification is determined by the relationship sign-ground-object, and Peirce went all the way in his analysis. Just for the fun of it, here are the complex nuances he goes into: