These terms—“signifier” and “signified”-- used by Ferdinand de Saussure to discuss his theory of linguistics (actually a philosophy of epistemology—how we know things--differing from Plato’s), begin the discussion of “the distance between the symbol—word, gesture, etc.—and the actual phenomenological object it (temporarily and imperfectly) stands for in communication.” Other modern linguistic theorists of the 20th century, such as Jacques Derrida and Noam Chomsky, and the speech act theorists (Stanley Fish, Edmund Husserl, John Searle, etc.), all treat the terms in their discussions.
The central point of examining these two terms is to free the “Ding an sich” (the thing itself) from the various imperfect signifiers that can refer to the thing. A “tree” is not the same thing as a living, growing plant differing from grass or a flower in various ways—“tree” is a signifier in the English language code, an utterance or a mark on a page, used in context to convey the real object or class of objects. While this seems fairly obvious, it is a mistake we make every day—“Dad” is a “signifier” whose “signified” differs with each of us, and whose use is limited to specific circumstances. The same is true of such noncorporeal entities as “freedom,” “hope,” and “belief.” Donald Hall puts it this way: “The definition of a word is another word.” The main distinction is that the signified is fairly stable in three dimensions or in the abstract, while the signifier is a member of a whole code, and changes with time (diachronically) and context (the signifer “pot” has dozens of “signifieds” in its history, for example). In communication, then, we must not give the signifier more power than it deserves (propaganda, political slogans, advertising, etc. all take advantage of the distance between “signifier” and “signified).” In literature, the author, especially the poet, usually counts on the distance to add layers of meaning to his or her words.
De Saussure characterized signs as a relationship between "concept" and "sound" to use de Saussure's words signified and signifier. Saussure called this relationship of signified to signifier a linguistic sign. The sign, for him, is the basic unit of communication. The linguistic sign is constituted by the structural relationship between the concept (e.g., "house"---the signified) and the sound of the word "house" (signifier). A language is essentially composed of such structural relationships, and the study of language is the study of the system of signs that express ideas.