Mid 20th century; Associated with French & German philosophical traditions rather than with English traditions. It has its origins in the Danish philosopher and theologian Kier Kegaard who reacted against the excess of rationalism of Hagel. For Hagel all oppositions can be reconciled and ultimately understood with an all embracing objective view of the universe. In opposition Kier Kegaard emphasised the importance of the individual life. In particular we are not simply subjective reaction two an objective uniformity. And that is because we have ethical questions which involve us in asking how we are to lead our lives. We deceive ourselves if we believe that personal anguish can somehow be resolved or removed by adapting ourselves to an objective or impersonal view of the world.
Kier Kegaard believed that we cannot take our existence as "real subjects" for granted. We must develop if we are to achieve our full potential as individuals. The question then is how can one become an "individual"? Certainly not by acquiring more factual knowledge, instead we have to engage the will. It is by making choices and commitments (e.g. marriage) that enable us to from our own identity. He writes "It is impossible to exist without passion." by which he mean that it is only by engagements which arouse the passions that we can gain a sense of our own identity and so become an "existing individual".
Karl Jaspers and Heidegger are two who developed these ideas, but it was the French writer and philosopher John Paul Satre who popularised these ideas. For Satre existentialism "means" that "existence precedes essence." Now this is very different from, for example, starting with a particular view of what it is to be human (i.e. in "essence") and defining the individual in terms of this pre-existing notion. Thus for example he says that to say that human beings have "fallen" and are in some why essentially sinful or evil is to start with the essence and apply it to existence. For Satre the reverse is true. The individual ego defines and develops itself through choices and in this way constructs an identity for which the individual human being is entirely responsible.
Satre introduces the idea of "bad faith". All of us, he says, practice it. We practice it when we refuse to acknowledge that we are acting in a certain way and therefor have responsibility for our actions. One of the examples he gives is of a girl who refuses to acknowledge the boy is holding her hand, insisting instead that they are talking about philosophy. If we are responsible for our own self construction as it where, then we cannot seek consolation in alternative views of human nature, be they genetic or in some way Idealistic (i.e. Platos Forms). Suffice it to say Sartres existentialism was in effect scorned by English philosophers trained in the close analysis of language - as Moore did to arrive at his naturalistic fallacy. In reply Satre emphasised his philosophy was concerned with real people in real situations. It is certainly not an irrelevance that Satre was a writer of fiction and it is in his novels and plays, and his political protests, that he may well become known in the future.
This an early novel was called nausea, and it is about a young philosopher seeking to come to turns with the anguish of personal choice in a world that appears harshly indifferent. And indeed probably existentialism is better as a literary theory than a philosophical system.