What's the difference between 'langue' and 'parole' according to Ferdinand de Saussure?
Parole refers to the individual language acts which occur when anyone audibly voices letters, words, sentences, etc. Parole is the physical manifestation of speech. Langue is the abstract system of principles language out of which acts of speech (parole) occur. Consider the analogy that the game of chess are the langue and the individual moves of chess itself comprise the parole.
Writing, like speech, is an example of parole because each act of writing a letter, word, or sentence is similar to voicing a letter, word, or sentence. Philosophers such as Jacques Derrida have made this agrument. For Saussure, however, writing is more systemic. In other words, he sees writing as the visual system of communication (speech). Writing is the categorized system of speech acts and sound-images.
Language is a system of signs that express ideas, and is therefore comparable to a system of writing, the alphabet of deaf-mutes, symbolic rites, polite formulas, military signals, etc. But it is the most important of all these symbols.
Saussure's wording can get confusing because sometimes he uses "language" to refer to writing, speech, and then writing and speech. Just remember that parole is speaking and langue is the abstract system of principles language.
Langue represents the “work of a collective intelligence,” which is both internal to each individual and collective, in so far as it is beyond the will of any individual to change. Parole, on the other hand, designates individual acts, statements and utterances, events of language use manifesting each time a speaker’s ephemeral individual will... (John Phillips, National University of Singapore)