SEMIOTICS
The study of signs.
French linguist Ferdinand de Saussure formulated his theory of language.

SIGN- 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 other definitions 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.

A 'signifier' - the form which the sign takes; and
The 'signified' - the concept it represents.

Signs can take many forms.
'Nothing is a sign unless it is interpreted as a sign'
(Peirce 1931-58, 2.172).
Anything can be a sign as long as someone interprets it as 'signifying' something - referring to something other than itself.

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

ICON
An icon looks like its signified.
For example computer icons, which popularize the word, or the pictographs 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.
A characteristic of the icon is that by looking at it, we can get information about its signified.

INDEX
An index has a causal connection to its signified. A key to understanding indexes is the verb "indicate".
A directly perceivable event that can act as a reference to events that are not directly seen, or in other words they are something visible that indicates something out of sight. You cannot see a fire, but you see smoke and that indicates that a fire is burning.

SYMBOL
A symbol represents something in a completely chance relationship.
The connection between signifier and signified depends entirely on the observer, or what the observer knows.
Symbols are subjective, words are an example of symbols.
As a spoken word or group of letters, they are only linked to their signified because we decide they are. The connection is neither physical nor logical, words change meaning or objects change names over time.
Symbols are ideas, and whenever we use one we are only pointing to the idea behind that symbol.
A symbol can rarely tell us anything more about its signifier than we already know.