Most of my friends are atheists and they would simply describe atheism as simply not believing in any deities/gods of any kind. That’s it.
When I was younger, 'atheism' meant belief that the proposition 'God exists' is false. I think that's how it's still typically used in the philosophy of religion.
Then in the 1990's, I started seeing aggressive promotion of the idea that 'atheism' simply meant 'lack of belief in God'. The first place I remember seeing it promoted that way was on the old usenet group alt.atheism. (A collection of loud and nasty idiots if I ever saw one.) Apparently it's based on a distinction invented by Anthony Flew between 'strong atheism' ('God exists' is F) and 'weak' atheism (I don't believe in God).
Rhetorically, weak atheism seems to many atheists to have two advantages:
1. It allows them to argue that atheism is the default condition for human beings. Babies are born without belief in God, hence arguably are 'weak' atheists, born in pristine atheist innocence. The goal becomes to prevent their corruption. (Shades of Rousseau.)
2. It allows them to insist that atheism bears no burden of proof. While belief in God must be forced to produce justification acceptable to the atheist (an impossible task), there is no need for atheists to produce any argument for atheism that's convincing to theists or to anyone else.
Frankly, I find both of those to be rather dim and 'unclear on the concept' at best. Disingenuous and intellectually dishonest if we're less charitable.
They dislike that there are atheists trying to create a religion out of disbelief.
If we start with 'weak' atheism, with the idea that atheists simply lack belief in God, we find that virtually no atheists are content to stop there. They will often insist that theism is responsible for all sorts of craziness and crimes that atheism isn't guilty of. Atheists will insist that there's no good evidence for the existence of God. They will insist that belief in God is equivalent to belief in the Easter bunny. They will insist (as W4U has done) that one can't be a theist and an intellectual. And on and on.
Chapter 8 of Richard Dawkins'
The God Delusion is entitled "What's Wrong with Religion? Why be so Hostile?" One of the chapter sub-titles is "How 'moderation' in faith fosters fanaticism". I think that it's obvious that a lot more is happening with Dawkins than mere 'lack of belief in God'.
You can label it whatever you like, but how can atheism be “extreme?”
How would I interpret the phrase "extreme atheism"? We can start with the mere lack of belief in God. That might arguably be a minimum qualification for being an atheist. But the more of what we might call 'auxiliary assumptions' we pile atop that minimum condition, and the more hostile and dismissive those new assumptions start to be towards other people's beliefs, the more 'extreme' the atheism gets. We move away from simply not believing in God to an ideology that sets itself up in opposition to any sort of religious belief. Eventually, out there at the extreme end, we end up with the mass murder.
Obviously atheists aren't unique in that regard. We can make similar observations about theistic belief, or about political adherence for that matter (which is probably the worst offender in our time). All of them appear to be forces for good or at least seem innocuous, but can turn nasty and can even result in terrible crimes when allowed to run amok.
It's the human condition, I guess. Atheists can't claim any kind of special immunity from it.