This isn't so much a "rant" as a simple clarification for those who don't know. In the atheist community, this subject gets brought up often. In the religious community, most of them don't even know there IS a difference, nor do they care (one faithless heathen is just as good as the next). Since this subject gets brought up often and since I'm tired of having to type the same things over and over, I thought it would be useful to have an in-depth explanation that I can link back to. First off, let's define some terms.
Deism - The belief that a god or gods exists. Period. Deism goes no further than that. It is not a belief in a personal god, an interactive god, etc. It is simply the belief that some powerful supernatural being exists and most likely created the universe. This may or may not also include the belief that this being created all life.
Theism - This is a step further than Deism. Theism is the belief in a personal god. The god of a religion. It's the belief that a powerful supernatural being not only exists, but that it cares about us and what we do. The word literally means "belief in a god." Most religious people are theists.
Atheism - The lack of a belief in any god or gods. The prefix a- is used in the same way it's used with words like asexual, amoral, atypical, etc. to mean "without" or "lacking." So a-theism literally means "without belief in a god" or "lacking belief in a god."
Gnosticism - Having knowledge of a god or gods. If the story of Moses were true, Moses could be said to be a gnostic, not a theist, since he would have had direct contact and personal experience allowing him to know for certain that a god exists.
Agnosticism - Though it's pronounced "ag-nos-ti-siz-um" the word is actually "A-nos-ti-siz-um" (silent g). Like with a-theism, we see the use of the prefix a- meaning "without" or "lacking." So the word literally means "without knowledge of a god" or "lacking knowledge of a god."
And now the important part.
Now that we've correctly defined the basic terms, the difference between Agnostics and Atheists should be clearly evident. Sadly, the common misunderstandings relate to the use of incorrect definitions for some of these terms (specifically, agnosticism).
The difference is that an agnostic is someone holding to the claim that they don't KNOW if there is a god or gods. An atheist is someone holding to the claim that they don't BELIEVE in a god or gods. The difference is one of belief versus actual knowledge.
The mistake that people make (most commonly) is by assuming that these two terms address the same question. They do NOT. Theism or Atheism addresses the question "What do you believe to be true?" while Gnosticism or Agnosticism addresses the question "What do you know to be true?"
I am gnostic about the existance of my genitals. I KNOW they're there. I am agnostic about the existance of your genitals since I have not actually seen them and do not have KNOWLEDGE of them. But at the same time, I could be said to be a theist about your genitals since I do BELIEVE that you have them and that they exist. While I might not KNOW, I do BELIEVE it's the case. Now, I also happen to be an atheist when it comes to your mother's testicles. I lack a belief that your mother has testicles, because she is a female. However, I also do not have direct knowledge that she lacks testicles, therefore I am also agnostic about them!
The point of that disturbing little example was to show how one can, in fact, be both agnostic and atheist. They are NOT exclusive labels since they do NOT cover the same thing! Most people who call themselves "atheists" are, in fact, agnostic atheists. While I do not believe that there is a god, I also could not possibly say that I know there is no god. No atheist can reasonably say that. I lack a belief in a god, so I don't think one exists, but I also lack the direct knowledge that this is the case.
The main confusion is that the term "agnostic" is used in modern colloquial speech to mean a "middle of the road" position. When people say "I'm agnostic!" they're saying that they don't believe in a god, but also don't believe that there is no god - they simply don't know, so they take a position "sitting on the fence."
The trouble is that, unbeknownst to these people, they are actually atheists too! Agnosticism, remember, answers the question of what you KNOW, not what you BELIEVE. It's fine to say "I don't know" but to say that you don't have a personal belief (even a tiny irrational gut feeling) is absurd. The question put forth by theism/atheism is "Do you believe in a god?" If the answer is anything other than "yes" then you ARE an atheist because you DO lack an affirmative belief that a god exists. Agnostic does NOT mean "I don't know so I'll stay neutral." Brother, NOBODY actually KNOWS so we're ALL agnostic (even those of us on this planet who claim otherwise).
It's perfectly acceptable to continue using the term "agnostic" incorrectly (plenty of people do it, no biggie), but it's not acceptable to be ignorant of your mistake. Now you have been educated. Put it to good use.