Humanism is a lifestance which embraces the natural and rejects the supernatural. This means that all humanists are atheists, and also means that they promote science and rationalism. Humanism also promotes human rights, and takes strong moral stances on a variety of issues. Not all humanists agree with each others morals, but they should all work from the basic starting point that humans are all equal and deserve certain basic rights.
Secularism is a separation of church and state. It is a stance held by both the religious and non-religious, and is important to ensure equality and stop religious discrimination. A secularist is someone who takes this view, and a secularist politician would create policies that allow both freedom of religion and freedom from religion, that is, people should be able to practice their religion (so long as it doesn’t harm others) and people should not be forced into situations where they have to engage in someone else’s religious practices. Most entities are in fact secular, by virtue of having nothing to do with religion – for example, a normal trip to the supermarket will be quite secular. If the supermarket printed bible passages on its receipts that would be non-secular.
Atheism is an absence of belief in Gods. It is separate from anti-theism, which explicitly states that religion is in some way “bad”. Atheism is often criticised of being a religion, but it is as much a religion as baldness is a hairstyle. There are two main sorts: to believe there is no God, and to fail to believe there is a God, strong and weak atheism respectively.
There are many kinds of agnosticism. Firstly, those who have not considered the answer to a question must be agnostic about it (de-facto agnosticism). There are those who believe the God-question is fundamentally unanswerable, those who believe there is not yet enough evidence either way, and those who refuse to try to answer the question until a sufficient definition of “God” is presented. Agnostics often reject certain types of God, particularly those types which are said to interact with the world.
Skepticism is something we employ whenever we are given new information. If it seems unlikely to be true, we may reject information until further evidence is given. People in the Skeptics movement tend to try to persuade others to be more skeptical about their beliefs, particularly with regard to alternative medicine, UFOs, and, indeed, Gods.