Existentialism is "the philosophical theory which holds that a further set of categories, governed by the norm of authenticity, is necessary to grasp human existence."  In other words, existentialism deals with human existence. It is considered more "a movement of the past", but is also considered an attitude of one's thought. Existentialism has had a profound affect on philosophy and theology over the last 50 years.
The first philosopher to use the term was Soren Kierkegaard (1813 - 1855), commonly called the "father of existentialism", although the term was explicitly adopted as a self-description by Jean-Paul Sartre. 
Existentialism has also been recognized by it's slogan, " Existence precedes Essence." This means that we have no predetermined nature or essence that controls what we are, what we do, or what is valuable for us. Further, we are radically free to act independently of determination by outside influences. We create our own human nature through these free choices, and we also create our values through these choices.  The following illustration may help:
Precursors of existentialism
- Soren Kierkegaard - Kierkegaard died too soon to be a part of the existentialist movement, and would have rejected many tenets of Sartre's existentialism. Yet, he was one of the first philosophers dealing with the problems of human existence in ways recognizable as forerunners of Sartrean existentialism.
- Friedrich Nietzche - Like Kierkegaard, Nietzche died too soon to be part of the existentialist movement, and, in many ways differs from the existentialism we know. Yet, his work is precursor to many of the developments in later existentialist thought.
- Martin Heidegger