Ferdinand de Saussure is responsible for one of the most dramatic intellectual shifts in the 20th century. It was de Saussure who moved to world from an essentialist perception of reality to a representational and structure dependant one. In a sense, de Saussure's thought is another step away from our ability to know anything about the world following Kant and Hegel's. de Saussure left after him the conclusion that, since everything is structure related, we can never have any stable and essential knowledge of anything. That there is nothing which is absolutely "there", only something which is represented in a system dependant manner.
Synchronic linguistics, rather than diachronic one, detaches language from the historical progression of the world, which leads to de Saussure detaching the lingual sign from its referent. de Saussure's distinction between Langue and Parole sets language is first and foremost a cultural-social mechanism which exists outside single individuals. de Saussure's notion of the lingual sing as being composed from the signifier and the signified "marginalizes" the actual referential world in favor of its symbolic representations. This argument by de Saussure led to serious question regarding the relation between the signifier and the signified which haunt western intellectual tradition to this day. One of deSaussure's key notion were in regards to the nature of the linguistic sign. de Saussure's ideas regarding the arbitrariness of the lingual sing gave rise to the understanding that the relation between language and reality, between the signifier and the signified, is socially constructs. Many a things have been signed off since de Saussure as being socially constructed. Other important concepts and considerations that were introduced by de Saussure are those of paradigmatic and syntagmatic.
de Saussure view of the language as a system or differences is fundamental for the subsequent structuralist movement which sought to map those system of difference which constructed social reality. This is partly why de Saussure was and still is such a huge influence and founding father of 20th and 21st century cultural studies and critical theory.