When Saussure examines language as a logical phenomenon that can be treated as a science, like any other artifact, he divides his study into two kinds: Synchronic comparisons of languages (meaning how languages compare to each other at any given point in time) and Diachronic study (meaning a study of how languages change over time—a historical examination of the influences of culture, travel, etc. on the way languages “work”—how they form rules, how they change forms (“morphology), how they grow and how they become “old”, etc. Just as a doctor, for example, will study the various organs in a patient at any given appointment, and also examine how the patient changes over time, all sciences divide their studies this way. Saussure saw language as just such a “patient” or “specimen.”
As discussed in the prior answer, Saussure was interested in introducing scientific rigor to the study of language, with specific emphasis on languages in their current form. In so doing, he not only separated the actual expression of language from the abstract rules and structure of language, he also drew a distinction between the study of language as it is used by current speakers and study of the historical development and context of languages. The study of current language use is called synchronic linguistics, and the study of the historical aspects of language, and the relationships between languages, is called diachronic linguistics.
Before structural linguistics, the school of linguistics of which many people consider Saussure the father, the primary focus of linguistics was philology, which is the study of the structure and historical development of languages, as well as the relationship of languages to each other. In other words, the study of language before the advent of structural linguistics was primarily diachronic in nature, focusing on the history and relationships of particular languages more than on the how people used language.
Saussure was among the first scholars to separate the study of the historical development and relationships of a language from the synchronic study of the extant languages themselves, as well as language as an abstract system. He was interested more in how people actually used language (parole), and in the abstract system of rules and conventions of language (langue) than on how the languages came to be and how they were related. He also believed that it was through the synchronic study of language that we could understand what language really is.
This shift in focus from diachrony to synchrony in the study of language marked a change in linguistics, moving the field away from philology and toward the study of speech and the psychological aspects of language. Saussure was at the forefront of this shift, and his separation of the study of language into synchronic and diachronic helped create a way of understanding the study of language that colored linguistics throughout the 20th century.