Existentialism is an attempt to find meaning in life when there is cause to despair.
Why would anyone believe that there is cause to despair?  The Cambridge Companion to Existentialism
 refers to reasons, as it interacts with the thoughts of such 
existentialists as Soren Kierkegaard, Friedrich Nietzsche, Martin 
Heidegger, Jean-Paul Sartre, Simone de Beauvoir, and Maurice 
Merleau-Ponty.  Reasons for despair can include arriving at the 
conclusion that God is dead, the inevitability of death, life not going 
as one believes it should, the absurdity or apparent meaninglessness of 
life, the feeling that one’s limitations hinder one from improving his 
or her situation, or fear of being judged by others.  Existentialist 
solutions to this despair include having goals, creativity, and making a
 conscious decision to accept life as it is.  Christian existentialists,
 such as Kierkegaard, regard faith in God as a solution: a person takes a
 risk by committing his or her only life to God.
My stereotype of existentialists was that they were moody and 
depressed, felt alienated from the world around them, and were more 
interested in the individual search for meaning than the well-being of 
society.  I wanted to read about existentialism because I was hoping 
that I would find in it a kindred spirit: yes, I am concerned about the 
well-being of society, but I am also moody and brooding, and I have 
often felt alienated.
While my stereotype of existentialism is not totally off the mark, 
there is more to the story.  Some existentialists are more hopeful than 
others.  Sartre, for example, was rather pessimistic about human beings,
 thinking that they used others for their own ends.  And yet, Sartre was
 very concerned about the well-being of society: Sartre leaned towards 
Communism, yet he became disillusioned with it on account of Soviet 
oppression.  Sartre also was critical of racism and colonialism.  I 
remember an episode of Family Ties in which Stephen Keaton was 
debating with his daughter Jennifer about whether or not Kierkegaard 
addressed the social problems of his day: Jennifer’s stance (which the 
show implied was correct) was that Kierkegaard believed that concern for
 politics detracted from one’s spirituality.  Stephen told Jennifer to 
go to her room!  Jennifer may have been correct about Kierkegaard, but 
it does not surprise me that there were existentialists who believed 
that politics were important, for existentialists believe in human 
freedom, and political systems do have an impact on that, for good or 
for ill.
In addition, it seemed to me as I read The Cambridge Companion to Existentialism
 that many existentialists valued community.  Yes, Nietzsche placed a 
value on non-conformity.  Yes, Sartre said that hell is other people!  
Yet, there seemed to be an acknowledgement among many existentialists 
that who we are is based, at least in part, on our community—-where we 
are.  There also appears to be some hope that community can be a 
solution to one’s existential crises, on some level.