As a philosophy, existentialism is associated with Sartre who believed that humans are free ("condemned to freedom") and that freedom brings responsibility.   He investigated the basic essence of what it means to exist as a human.     The basic existential problems existentialists write about and ponder upon are  

                1. the transitory nature of life

                2. time and eternity

                3. difficulty in communication and contact with others

                4. individual's sense of loneliness and alienation

                5. mysteries of identity

Camus was a friend of Sartre and was also an existentialist. Camus stated his belief thusly: "humanity has to resign itself to recognizing that a  fully rational explanation of the universe is beyond its reach;  in that sense, the world must ultimately be seen as absurd."