Well, my knowledge is limited, but I'll do my best to explain.
Every source that I've read in regards to existentialism always says it is hard to define, because there are, apparently, many types of the philosophy. But so far, I've only found two different kinds: theistic existentialism, and atheistic existentialism. Apart from the factor of God, the two are different in that TE measures a man's worth by his response to God, while AE measure's man's worth by his response to existence.
But for the concepts that knit the two together...
- Sartre coined the famous phrase, "Existence precedes essence." At first, I had a difficult time understanding this idea, but that's probably because I never understood what "essence" was. But it is rather simple once you get past that silly block.
Basically, it means that humans exist before they have a purpose or meaning. From conception, you did not have a pre-determined goal in life. Where as the Westminster Confession of Faith (a Christian document) answers the question, "What is the chief duty of man," with, "To enjoy and glorify God forever," existentialists say humans have no prior essence.
Now you may reasonably ask, as I did, "Does this apply to all things?" Clearly not. As Sartre explained via papercutter, A papercutter has an essence before it exists. When a man needs to cut paper, he sets off to create the proper instrument. He thinks about how to create, what materials to use, and so on and so forth. Then, he creates the papercutter, and uses it to fulfill its purpose.
But humans have no essence, and that is because they have no Creator. The TE takes offense to this, and although they believe existence precedes essence, they attempt to solve the contradiction by saying humans exist before their essence, and the essence is found in their response to God. However, that doesn't solve anything, because if God existed, it would be an insult to the Creator to assume that He would create something without any thought or purpose.
Thus, existence precedes essence. And lest you think, "So life is meaningless, why should I live it," the existentialist has an immediate answer: You are free to choose your essence, which leads to the second tenet of existentialism.
- Again, Sartre defines it for us, "We are condemned to be free." Since we as humans ultimately have no pre-set significance, you and I are free to choose what we want to be. That isn't so hard to understand, right?
Sadly, the vast majority of humans hate being free, and that is because with freedom comes responsibility. If you choose to do something, and that freely, then you alone are responsible for the consequences. Many people try to avoid this freedom in numerous and divers ways. For example, a person who loves to eat might say, "I inheritated some genes from my parents." Therefore, it isn't their fault that they are obese, blame the parents. Or if a rapist tried to argue, "It's just who I am," they are assuming they have a nature that somehow disposes them to rape. Another attempt to avoid responsibility is to play off environment. If a child fails a test at school, it isn't his fault because he stayed up until midnight, and his dog died the next morning, and his teacher really hates him. If he failed, it's because his environment made him fail, or so he would have you to believe. But the existentialist denies all these things. We are free to do almost whatever whenever we want, if we so choose. But you see, we don't always choose. We just let someone or something else choose for us, and then you have engaged in what is called "bad faith" or "self-deception," both ideas which I need to learn more about, but basically it is the attempt to rid yourself of freedom and therefore responsibility.
Now this all may sound harsh. What about the lady who had her legs amputated because she got in a tragic car accident? Surely she isn't responsible for that? The existentialist thinks so. She chose to get in the car and go driving, and the car accident was the result. She is responsible for freely getting in the car, and nothing else. Well, maybe she was going to get groceries for her three children. That doesn't absolve her of responsibility for her consequence. She could have chosen to go at 4:30, or 5:00, but she chose 3:00, and that led to the car accident. No one made her choose 3:00 except herself.
But you see, she is still free, even with her legs amputated. She is free to figure out how to move on from this experience. Will she be bitter the rest of her life, or will she use this moment to warn others of the dangers of driving? It's up to her, because she's free to choose, and whichever she does choose, she is responsible for the outcome.
"Wait a minute," you might say. "Am I absolutely free? Does that mean if I were to close my eyes and wish myself to Hawaii, I am free to go there?" No, not necessarily. What the existentialist means by "free" is "free within a certain system." Let's take Sartre's example of the artist: An artist has red, blue, and yellow for her colors. Now, is she free to make a green tree with just the red paint? No, no, not that kind of free. But she is free to comine blue and yellow to make green for her tree, if she wants to have a green tree. Maybe she wants a purple tree, which is entirely up to her.
- Life is absurd. That is Albert Camus, another French existentialist who lived alongside Jean-Paul Sartre, and "life is absurd" is his major contribution to existentialism. How is life absurd?
By that, Camus means that we live despite knowing we will die. Have you ever noticed that? If I'm going to die anyway, I might as well commit suicide and get it over with. But Camus rejected that belief, and tried to answer it. I haven't studied Camus as much as Sartre, so I won't be very informative for you readers, but from what I have read, this is what I know.
Try holding your breath. Go as long as possible. Is your heart beating faster? Chest pounding? Want to give in? Okay, take a breath, I don't want you to pass out. What just happened? Your body wants you to live. Your heart was trying to get oxygen to your body, but if you have no new supply, it pumps harder. Your heart doesn't want you to die, because if it did, it would stop beating. And yet it keeps going, and going, and going, and going. If you held your breath long enough, your brain would have shut down, causing you to pass out and start breathing again for the sheer reason of staying alive. Despite knowing you will die, your body strives to live. Life is absurd. So how are you going to deal with it? Are you going to commit suicide, or are you going to live despite this morbid knowledge? And how are you going to live? You are the author of your life. Write an interesting story while you can.
I'm sure there are plenty other tenets of existentialism which I am forgetting or haven't studied yet, so my apologies if that is the case. I hope I have made as accurate as possible an interpretation of French existentialism.