Atheism is the rejection of belief in the existence of deities.
Agnosticism, on the other hand, is the view that the truth values of the existence of God, the divine, and other supernatural things are unknown and perhaps unknowable.
The two are very different, but often confused in larger society, so we'll explain them both on this one page.
Atheists believe that there are no deities.
The first people to identify themselves with the word atheist arose in during the Age of Enlightenment in the 18th century. Since conceptions of atheism vary, it is difficult to accurately estimate the numbers of atheist individuals in the world. Some polls have have shown around 20% in europe, even 60% in China, and 8-12% worldwide.
There is no single ideology or set of behaviors which atheists adhere to, so it is not a lifestyle descriptor as many religions can be. Rather, any type of non-religious person may be an atheist. Most atheists hold that atheist is a more parsimonious world view than theism -- that is, it requires less assumptions -- and therefore the burden of proof lies on theists to provide a rationale for theism.
Most rationales for not believing in deities come from lack of empirical evidence for deities, and there is also the Problem of Evil, The Argument from Inconsistent Revelations, the rejection of concepts that cannot be falsified, and the Argument from non-belief.
- The Problem of Evil: this refers to the question of how evil can reconcile with an omnibenevolent deity. The question has struck people with confusion across generations: if there is a God and he/she/it is good, why did it create evil?
- The Argument from Inconsistent Revelations: this is the argument that God cannot exist because of how many revelations of God across the world have been conflicting an mutually exclusive.
- The Argument from Non-belief: similar to the Problem of Evil, this argument points out a contradiction in the existence of God in a world where people do not believe in it. More specifically, "Why would God create a world in which many people do not see/believe in him/her/it?"
Some people consider themselves Darwinist. The term has seen wide variations since its first use. At first and in its purest meaning, "Darwinism" is a theory of natural evolution developed by Charles Darwin stating that species arise through the natural selection of surviving ancestors: survival of the fittest.
In the mid-1800's, Social Darwinists argued for anti-philanthropic political agendas. Eugenics -- a controversial set of aims and beliefs that focuses on improving the quality of genetics of the human population -- was founded on social "Darwinism".
The term Darwinism is commonly used today in the United States, especially by Creationists, as an epithet to attack evolution (a scientific theory) as though it were an ideology (an "ism") of atheism.
Agnostics believe that the truth value of metaphysical and religious beliefs is unknown and probably unknowable.
According to the philosopher William L. Rowe "agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify either the belief that God exists or the belief that God does not exist" 
The words "agnostic was coined by Thomas Huxley in 1869, but agnostic thinking goes as far back into recorded history as the 5th-century BC. 
A really quality explanation of agnosticism comes from Thomas Henry Huxley:
"Agnosticism is of the essence of science, whether ancient or modern. It simply means that a man shall not say he knows or believes that which he has no scientific grounds for professing to know or believe. Consequently Agnosticism puts aside not only the greater part of popular theology, but also the greater part of anti-theology." 
"Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle ... Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect do not pretend that conclusions are certain which are not demonstrated or demonstrable." 
AtheismUnited website list: a HUGE list of websites and directory of organizations to do with atheism.
godisimaginary.com: the front page is a list of 50+ Atheist proofs
The Skeptic"s Annotated Bible: this is literally an online bible annotated with counter arguments. It highlights contradictions and paradoxes.
American Atheists: a politically centered online atheist hub with articles and more.
SecularWeb: "A drop of reason in a pool of confusion"
New Advent's Refute: The catholic encyclopedia attempts to refute agnosticism.
Debate: Evolution vs. Creation - Ken Ham vs. Bill Nye: The "Science Guy" himself slams leading creationist Ken Ham in objective debate. 2.5 hr
Science Can Answer Moral Questions: a TED talk by Sam Harris about his concept of the moral landscape.
Niel Degrasse Tyson and Richard Dawkins have a chat: two leading skeptics of our day have a conversation in the Hayden Planetarium in New York. 1hr 22min
The God Delusion Documentary: A 1.5 hr documentary adaptation of Richard Dawkin's book The God Delusion
15 Things to Never Say to an Atheist: an atheist talks about his lifestyle and misconceptions dealt with by Christians. Targeted toward religious thinkers. 11 min 20 sec.
Religulous: a documentary by Bill Maher full of sarcasm, very critical of Religion. 11 parts about 10 minutes each.
Why I am Agnostic: an interview with Neil Degrasse Tyson, the world's leading spokesman for astronomy/astrophysics.
Atheist or Agnostic? Neil Degrasse Tyson Dissociates and clarifies between Atheism and Agnosticism. "I don't play golf. Is there a word for non-golf players?" 3 min 48 sec
Creationism is not Appropriate for Children: a quick thought by Bill Nye on this very relevant debate to Atheists and Theists alike. 2 min 30 sec.
Mystic Vs. Scientist: Watch a fascinating conversation/debate between a well-known mystic and scientist. Jaggi Vasudev and David Eagleman speak on a multitude of subjects at Rice University, from memory and time to identity and the brain. ~50 min
The most beautiful experience we can have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder and marvel, is as good as dead. It was the experience of mystery, even if mixed with fear, which engendered religion. Our knowledge of the existence of something impenetrable to us, our perceptions of the profoundest reason and the most radiant beauty, which only in their most primitive forms are accessible to our minds - this very knowledge, this emotion, constitute true spirituality and religiosity. In this sense and in this alone, I am a deeply religious man. I cannot conceive of a God who rewards and punishes his creatures, or has a will of the kind that we experience in ourselves. Neither can I nor would I want to to conceive of an individual that survives physical death; let feeble souls, from fear or absurd egoism, cherish such thoughts. I am satisfied with the mystery of the eternity of life and with the awareness and a glimpse of the marvelous structure of the existing world, together with the devoted striving to comprehend a portion, be it ever so tiny, of the Reason that manifests itself in nature.
- Rowe, William L. (1998). "Agnosticism".
- Lloyd Ridgeon (March 13, 2003). Major World Religions: From Their Origins To The Present. Taylor & Francis. pp. 63
- Thomas Huxley, "Agnosticism: A Symposium", The Agnostic Annual. 1884
- Thomas Huxley, "Agnosticism", Collected Essays V, 1889