There are a lot of agnosticism resources on this site for beginners. There are articles on what agnosticism is, what agnosticism is not, and refutations of many popular myths about agnosticism.
Because people's knowledge, needs, and misunderstandings will change over time, the information presented here will also evolve over time. If you don't see something here that you think should be included because more beginners need to know about it, just let me know.
What Agnosticism Is
Agnosticism is the Absence of Knowledge of Gods: Although sometimes used metaphorically to denote a lack of commitment with respect to any given issue, agnosticism strictly taken means not claiming to know for sure if any gods exist. This is the definition for agnosticism in standard, unabridged dictionaries. Because of the use for "lack of commitment" other areas, many attribute that back to the question of gods' existence as well and conclude that agnostics are "uncommitted" to any position on whether any gods exist. This is a mistake.
Weak Agnosticism vs. Strong Agnosticism: Sometimes a distinction is made between weak agnosticism and strong agnosticism, an analogy to the distinction between weak atheism and strong atheism. A weak agnostic refuses to make any knowledge claim for themselves; a strong agnostic denies that any human could possibly know. So a weak agnostic says "I don't know if any gods exist or not." A strong agnostic says "no one can possibly know if any gods exist or not."
Agnosticism should be Philosophical: A person who is self-consciously agnostic is (or should be) agnostic for philosophical reasons derived from their epistemology and their ethics. Technically, though, a person doesn't have to have thought about the issues very much to be agnostic. They don't even have to care whether any gods exist or not — they can be completely apathetic about the question. The definition of agnosticism doesn't depend upon a person's reasons for their agnosticism
Agnosticism is Compatible with Religion: Being an agnostic doesn't necessarily mean that a person can't be religious. To the degree that a religion's dogmas include claiming to know that a god exists it will be difficult for an agnostic to be part of that religion. That's common to western religions, which may be part of why most agnostics in America don't attend religious services. In some religions, though, agnosticism can play an important role. That said, though, agnosticism itself is not a religion and cannot be a religion, just as atheism and theism are not themselves religions and cannot be religions.
What Agnosticism Is Not
Agnosticism is not a "third way" between atheism and theism because it is not mutually exclusive from atheism and theism. Agnosticism is about knowledge which is a separate issue belief. Agnosticism is thus compatible with atheism and theism — you can be an agnostic atheist or an agnostic theist.
Agnosticism is not just sitting on the fence or a failure to commit to something and it's not a suspension of belief. It is also not, contrary to what some might tell you, the only possible rational option. Agnosticism is not inherently undogmatic or rational; agnosticism may be held dogmatically and for irrational reasons. There is nothing in agnosticism that is inherently superior to atheism or theism.
Origins of Agnosticism
Agnostic musings and ideas can be traced back to the earliest Greek philosophers and has even played a role in western theology. Agnosticism should be treated as a respectable, reasonable philosophical position — at least, when held for respectable reasons. It should not be dismissed as a fad or as trivial.
The first person to use the word "agnostic" was Thomas Henry Huxley. Huxley described agnosticism as a method rather than a creed and even today some use "agnostic" to describe how they approach issues rather than as a position or conclusion. Robert Green Ingersoll was such a fierce proponent of agnosticism that it's now associated almost as closely with him as with Huxley. According to Ingersoll, agnosticism is a humanistic approach to knowledge which is superior to the traditional Christian approach.