Existentialism is a system of thought popularized by French philosophers Albert Camus and Jean-Paul Sartre in the mid-20th century but tracing it's roots to Danish philosopher Soren Kierkegaard in the early nineteenth century. One of it's signature characteristics are that it's meaning can fluctuate depending on the individual proponent, but there are resemblances which are generally regarded as binding these myriad writings together; among them are an emphasis on individual choice and responsibility, the overall absurdity of existence, and the relationship between subject and object. The root of the term Existentialism is 'exist', and the Greek roots for 'exist' are "Ek", meaning 'apart', and 'sisteri', meaning to stand. "Standing apart" is of special significance to Existentialism because of its emphasis on individualism.
The original Existentialist writer was a Dane by the name of Soren Kierkegaard. A deeply religious man, he wrote in reaction to what he saw as the bland, listless faith of his time. Christianity, he said, was not a fashion statement or something one inherited as a part of a group, but a deep personal commitment in the face of one's fears and reservations. In order to live authentically, he reasoned, one must make one's choice and stand by it, and separate oneself from the herd.
Friedrich Nietzsche would echo these sentiments a few decades later, but would add a virulently anti-Christian sensibility. He emphasized the soft complacency and conformity to convention of his age, and introduced the idea that the twin Greek dieties Dionysus and Apollo were the principal archetypes for human behavior. A philologist, he hearkened back to antiquity to demonstrate what he saw as the ideal human being, the ubermensch, meaning one who has cast off convention and lives boldly in pursuit of his desires. Will was the driving force behind human innovation, art, and progress, convention is the enemy of the will, and religion, Lutheranism in particular, is convention personified.
Martin Heidegger was a German philosopher writing in the early 20th century. He contended that the principal question of philosophy is that of the nature of being. His voluminous work, "Being in Time", outlines his characterization of the different modes of being. In order to live authentically, human beings must face the bare facts of their existence. Socrates' syllogism, 'all men are mortal', is a heart-rending truth that must be confronted. Also important is the nature of the mind, and that because of that nature one can never truly know other people. He called this facing of the harsh reality 'dasein', a German word meaning 'being-there'. This facing of reality is exhausting, however, and people eventually revert to 'in-der-welt-sein', or 'being-in-the-world', allowing everyday cares like money, status, and relationships distract one from the ultimate truths of life.
Existentialism is often called a philosophy, and to be sure, its proponents are often philosophers, but they are just as often playwrights, novelists, and ministers. This is because, as a philosophy, it deals not in the abstract questions of the academy, but instead gets down to the brass tacks of existence and tackles the questions faced by everyone who has ever lived. It is a philosophy which can accommodate men of faith and atheists alike.
Albert Camus was a pied noir, a French-Algerian writer and playwright who developed two concepts important to Existentialism. The first is the idea of "Absurdity", and the second is the Myth of Sisyphus. Camus mused that, as one goes through life, one is faced with the reality that, although fairness and justice are concepts we learn about as children, we do not find them in the world at large, except in isolated cases. For Camus, men long for God in a godless world, and when our desire for goodness meets the cold, harsh reality of an indifferent universe, the result is that we find it absurd. Absurdity is a feeling that we get when our ideas about the world are dashed against the rocks of the cruelty of life.
The myth of Sisyphus was an essay in which Camus describes the ancient Greek myth of Sisyphus, an arrogant man whom the gods punish eternally by forcing him to roll a boulder up a hill, only to have it roll down again. Camus asserts that, although the man's fate is a cruel one, the man is in complete control of his reaction to his situation, and that he can realize happiness if he loses himself in his work and creates his own meaning even when it seems that everything he does is meaningless.
Jean Paul Sartre was a friend and contemporary of Camus. A member of the French Resistance during the second world war, he was a staunch proponent of individual responsibility. His book, "Being and Nothingness", builds on Heidegger's concept of dasein and adds that authenticity requires that the individual embrace the weight and consequences of their choices because choices are the only things in life which provide meaning to their lives. If someone refuses to accept the consequences of their choices, and instead hides behind convention, he is said to be living in bad faith.
Other notable existentialists include Martin Buber, Paul Tillich, and Karl Barth, and writers who incorporate existentialist themes in their writing are Franz Kafka and Fyodor Dostoevsky, among others. For an excellent treatment of the philosophy, see "Irrational Man, A Study in Existential Philosophy" by William Barrett.