Wikipedia: Existentialism is a Humanism (L'existentialime est un humanisme) is a 1946 philosophical work by Jean-Paul Sartre. It is seen by many as one of the defining texts in the Existentialist movement.
In his text, Sartre says that the key defining point of Existentialism is that the existence of a person comes chronologically before his or her essence. In simple terms, this means that, although that person exists, there is nothing to dictate that person's character, goals in life, and so on. Only the person himself can define his essence:
Man first of all exists, encounters himself, surges up in the world - and defines himself afterwards.
Thus, Sartre rejects what he calls "deterministic excuses" and claims that all people must take responsibility for their behaviour. Sartre defines angst and despair as the emotions people feel once they come to the realize that they are responsible for all of their actions. He also describes forlorness as loneliness atheists feel when they realize that they are all alone, that there is no God to watch over them. This is associated with despair and angst.
The essay has been criticized by some for giving only a superficial overview of the themes of existentialism, and Sartre himself has called its undeveloped morality an "error".
"Existentialism is a Humanism" was also the title of a lecture he gave to a packed audience at Club Maintenant in Paris, on October 29th, 1945. The book is based on the lecture.