Semiology: language, langue, and parole
Saussure focuses on what he calls language, that's "a system of signs that express ideas," and suggests that it may be divided into two components: langue, referring to the abstract system of language that is internalized by a given speech community, and parole, the individual acts of speech and the "putting into practice of language".
While speech (parole) is heterogeneous, that is to say composed of unrelated or differing parts or elements, language (langue) is homogeneous, composed of the union of meanings and 'sound images' in which both parts are psychological. Therefore, as langue is systematic, it is this that Saussure focuses on since it allows an investigative methodology that is rooted, supposedly, in pure science. Beginning with the Greek word ‘semîon' meaning 'sign', Saussure names this science semiology: ‘a science that studies the life of signs within society'.
A popular view of language is that it is a natural organism, that grows and evolves in accordance with fixed laws and is not determinable by the will of humans. Saussure argued against that organicist view of language. Instead, he defined language as a social product, the social side of speech being beyond the control of the speaker. According to Saussure, language is not a function of the speaker, but is passively assimilated. Speaking, as defined by Saussure, is a premeditated act.
Read more about this topic: Course In General Linguistics