Existentialism is a philosophical school of thought that is primarily concerned with thoughts about existence and the questions and considerations that existing elicits. Existentialism rose to prominence in the mid-to-late 1900's, though inklings of existential thought can be found in much earlier philosophical works.
Existentialism is a philosophy primarily focused on the individual and the individual's existence, as opposed to many other philosophies that are primarily social in nature. It grapples with common questions that most people face: the meaning of existence, whether or not humans have free will, what it means for something to have value, and the place for self-knowledge in the scheme of existence. Existentialism, as well as providing many questions about existence, also arrives at conclusions about life. Some of the conclusions about life that are held in existential philosophy include the innate stressful and difficulty of making decisions, life and the things in it cannot and are not always rational, society as a construct is not natural, and wanting things of the word is pointless.
Existential philosophy mostly rejects the belief in an innate objective meaning of life, arguing instead that existence proceeds essences. This roughly means that one comes into existence, and then finds meaning within the life they have found themselves embroiled in. The philosophy also places great importance on actions rather than words, and in living life in-tune to one's own personal purpose. Existential thought and existential philosophy can be found in the works of many well-known philosophers. Kierkegaard and Nietzsche are often regarded of the father's of existentialism. They both wrote about existential ideas long before it was an accepted standard philosophical movement. In contrast, Heidegger, Sartre and Camus are often regarded as the men who brought existentialism into the forefront of the philosophical world. These men fleshed out the philosophy and molded it into the school of thought we know today.
Existentialism is truly a unique sub-set of philosophy rich with history and questions and concepts universal to all people.