"To live is to suffer, to survive is to find some meaning in the suffering."

Existentialism is a very intricate and complicated subject to address. Philosophy in general is. The very subject broaches the entirety of human thought and emotion beyond language, and so attempting to define it and understand it in the terminology of written words is nigh on impossible. Definitions stretch on for pages on pages; they transcend "Definitions" and become essays on the subject. Simplicity is destroyed, as even the most basic terms must be defined in terms of every other philosophical term out there, without being defined by characteristics of itself or defining itself with characteristics of others that also happen to be have the same characteristics. It gets pretty 'deep'. That being said, let's try to understand, simply, the philosophy of existentialism. As far as I can tell, existentialism falls into the philosophical category of human purpose, drawing purpose of life from the individual, with their own free will and soul. There is some supernatural or otherworldly aspect, not necessarily religious, but not definable in terms of matter, science, and causality alone. Philosophy is also difficult to deal with because, with its very complex nature, it becomes incredibly specific, and differing viewpoints on only a couple facets send a whole new philosophy spinning off into being.
For example, I share with existentialism the belief that purpose is begotten from the individual: you make your own purpose in life by your actions: not necessarily that every person has a specific self-created purpose to follow, but that if you wish to find purpose, look to yourself and mind, which means that people can very easily exist without a purpose, and very many in fact do not have purposes. I do not, however, subscribe to the illusion of free will, meaning that while there are no set purposes to follow, every person's purpose, as defined by their actions, being predeterminate, is already set. I also do not subscribe to the philosophically idealist views of existentialism [philosophical idealism and materialism are two opposites in philosophy: idealism is the belief in things that transcend the real world, things such as planes of existence beyond our own like heaven or hell, or that consciousness arose beyond matter and only inhabits matter by choice. Materialism states that there is only the real world, with only matter and scientifically explainable phenomena, and that consciousness has arisen from the highest possible state of matter.], so one might call me a materialist existentialist. But this still does mot encompass all the philosophical views I possess. In addition, I am a dialectic [the Hegelian philosophy of change or motion, especially in a social sense when applied to Marxism, that encompasses the drastic and sudden change from quantitative to qualitative fluctuation and vice versa, the unity of opposites, the negation of the negation, and so on], a Marxist [or scientific socialist, which really encompasses dialectical materialism, historical materialism, and Marxist economics], an atheist [which also sometimes, but not always, includes materialism], a hard determinist [which actually happens to be very difficult, but not impossible, to rectify with existentialism of any sort], a realist [not just materialism, but that everything we observe as the real world is actually real], a moral relativist [which just so happens to be morally iffy, which by its very nature I care little about], and so on.
However, of all the philosophical fields out there, the philosophies for finding meaning to life are by far the most complex, as they try to find an answer to the great question "Why are we here?", a question that has plagued conscious thought since its beginning. This is also the most difficult answer to accept [if an answer to the question is ever found]. Almost every belief system out there stems from some facet of this question, not only belief systems to answer questions about the world around, but also as to why we are the ones that can actually acknowledge it. Most belief systems were created to form false senses of grandeur and purpose in this universe, [before I go on I should specify that my views for the purpose of human life are that purpose is not actually tangible or given, it is a relative, subjective, and self-prescribed comfort] as people fail to accept either the truth that they have no given purpose or they fail to accept the responsibility of being given their own purpose to procure. Deep stuff.