Many people wonder what an existentialist is. A lot of existentialists don't know how to define the term, either. There doesn't seem to be a concise, clear-cut definition. Personally, I find that freeing. It allows for more variety in the ways we can apply the philosophy to life.
To me, existentialism is the belief that we are free to make choices. This freedom comes with a moral responsibility. It's not anarchical freedom. Our free will obliges us to discover what is right for ourselves and for the world at large. Then we are to act on it. Some associate existentialism with secular humanism, the belief that humans are the only organizers of the world and that we are governed by no supernaturally-sanctioned laws (and we are "free" from a deity as well). Some existentialists are secular humanists, but the two philosophies are not always combined. Christian existentialists (such as myself) believe that God grants us free will so we may learn and grow, and we ought to use it to serve Him. We serve and worship in a multitude of ways. Acts of kindness are acts of service because we engage in them for the greater good, and the greater good is synonymous with God. I don't view service to God as slavery, nor do I think God is a tyrant. The way I perceive it, we interact with God in the dynamics of a parent/child or teacher/student relationship. He lovingly guides us. We are able to err and to face the consequences of those mistakes, but only so we may learn and improve.
While I'm on the topic of existentialism, I can't help but think of my blogger name. It would be pretty interesting if Mattel actually sold an existentialist Barbie. She'd have to come accessorized with a mini Sartre book and a cute little bottle of Prozac.