Lately I have been reading a number of books and essays that are considered "existentialist". However, most people don't really know what "existentialism" is. Existentialists are often characterized as either paralyzed by a contemplation and a fear of action, or selfish and wantonly immoral. While it is difficult to define exactly what existentialism is because the term was not actually used until the 1950s, and its philosophers largely classified as such in a retrograde fashion, a generalized definition can be given.

http://www.anselm.edu/homepage/dbanach/sartreol.htm

Read the page at the above link.  If you find it interesting, please read the whole essay, Jean-Paul Sartre's "Existentialism is a Humanism".  It can be found at http://www.marxists.org/reference/archive/sartre/works/exist/sartre.htm
It is a very simplified explanation of the basic beliefs of the philosophy but does a pretty good job of it.