Atheism: I don't believe in any gods.Ignosticism is covered very well in Oolon Colluphid's award-winning book 'Who is this God Person Anyway?' Or at least, it would be if that book actually existed. Instead, in this universe, the idea of ignosticism was put forth by the Rabbi Sherwin Wine founder of Humanistic Judaism.
Agnosticism: I don't know if any gods exist.
Ignosticism: I refuse to take a position until "god" is defined.
Ignosticism is very closely related to theological noncognitivism which states that all religious language is purposely vague and cognitively meaningless. "God" is intentionally left as a vague idea because if you define something then it is possible to prove or disprove that it exists. Therefore "God" (and all things supernatural) live in a linguistic nowhere-land forever in search of a meaning. All of this is because no one has ever been able to come up with a positive definition for what "God" is. He (or She) is always defined by things that He (or She) is not. Ignosticism takes this position of theological noncognitivism and simply states that since no one can put forth a definition I will withold judgement until a definition is presented.
There is always that fourth non-religious position but it doesn't really warrant it's own blog post. Apatheism is just the position of "I don't know, and I don't care." Almost nothing is written about the topic because... well, yeah. It'd be interesting to know exactly how many humans fall into this category though.