One of the leading philosophical movements of the twentieth century, existentialism has had more impact on literature and the arts than any other school of thought. Focusing on the leading figures of existentialism, including Sartre, Nietzsche, Existentialism may be difficult to explain, but it is possible to communicate some basic principles and concepts, both regarding what existentialism is and what it is not. On the one hand, there are certain ideas and principles which most existentialists agree on in some fashion; on the other hand, there are ideas and principles which most existentialists reject - even if they don't then agree on what to argue for in their place.
It can also help to better understand existentialism by looking at how the various trends developed long before anything like a self-conscious existentialist philosophy was promoted. Existentialism existed before existentialists, but not in a single and coherent form; instead, it existed more as a critical attitude towards common assumptions and positions in traditional theology and philosophy.