Jerry Everard's Introduction's to Saussure ...
Ferdinand de Saussure introduced the notion of the arbitrariness of the sign. Historically, language has essentially been considered to be a naming process. Things (including imaginary things) were provided with words or signs to denote them. More importantly, early grammarians assumed that there was some necessary connection between word and thing, name and object. Saussure challenged that notion by pointing out the radically different names given to things in different languages. For example:
In other words, he pointed out that there is no a-priori connection between the word and the thing.
Saussure thus sees language as a system of differences, with no positive terms. If words stood for pre-existing concepts, they would all have exact equivalents. But in reality, different languages divide the world up in different ways.
Saussure differentiates between the sign, which is the totality of the sound/image (or signifier) along with what it denotes (the signified), and these two component parts considered separately.
Signs are defined by their difference from each other in the network of signs which is the signifying system. While the individual sign is arbitrary there is an important sense in which the signifying system as a whole is not. Meaning is socially produced and operates by convention. Thus the social construction of the signifying system is intimately related to the social formation itself. This has important consequences for the relationship between language and ideology - the sum of the ways in which people both live and represent to themselves their relationship to the conditions of their existence. Ideology is inscribed in signifying practices, through discourses, myths and representations of the way 'things are', or of what 'goes without saying'. Expressions which inscribe gender relations are a visible example of this.
Saussure suggested that language should not only be studied in terms of its individual parts, and not only diachronically (across time), but also in terms of the relationship between the parts synchronically (the way it works now as a snapshot of meaning relations). In this way, Saussure sought to recognise a language's structural properties as well as its historic evolution. Thus Saussure showed that language functions at each moment as a complete system at any given point in time, so he was able to examine the language system a-historically.
Saussure also draws the distinction between what he terms LANGUE, or the overall set of rules of language, and PAROLE - the individual instances of language use. The system is dynamic, so he was not trying to suggest that language structures are prescriptive, but rather that the rules may be inferred from the way language is used at any given point in time. So he was opening the way for descriptive linguistic studies, such as those of MAK Halliday, Noam Chomsky and other structuralist linguists.
His work may be followed up in the Course in General Linguistics which is a collection of his lectures assembled by three of his students in 1916.
Reference: Ferdinand de Saussure - Course in General Linguistics trans by Wade Baskin, McGraw-Hill: NY (1966)