Most of the time when you encounter the terms Atheist and Agnostic they usually sit next to each other in some kind of multiple choice question on which religion you subscribe to. Seeing as you'll need to answer the question, knowing what they mean – and their difference – is important.
First of all, neither Atheism nor Agnosticism are in fact religions, even though they end in -ism. In common speech they are used as two ways of describing a lack of religiousness in two different ways.
Athesits take a strong stand against all religions and gods. They don't think there is a god, any gods, spirits or any similar sentient and powerful force. Which is why they do not take part in any religion. They firmly do not believe in any of it.
Agnostics, however, aren't sure because they generally don't believe in any particular god or spirit. They often think that "there's something" but don't know much about details. Although they certainly may take part in religion, they're not very religious because they simply aren't sure of which one, if any.
Now that we know what being an Atheist or an Agnostic means, let's ruin that by realizing the above explenation is wrong. Well, it's correct in everyday usage, but technically, they're like apples and oranges – they can't be compared.
We're actually dealing with two different questions. One is "Do we think there is a god or not?" and the other is "Do we think we can know for sure or not?".
Regardless of how sure, or unsure, you are, if you think there's a god then you're a Theist, and if you think there is not then you're an Atheist. Additionally, regardless of your belief in a god or no gods, if you're absolutely sure about it then you're a Gnostic but if you aren't really sure then you're an Agnostic.
You believe there is a god.
You believe there is not a god.
You think the truth can be (or is) known.
You think the truth can not be (or isn't) known.
This means that you have to both be one of Theist and Atheist, as well as one of Gnostic and Agnostic. Of course, religious belief, or non-belif, is more complicated than this, but check out the following simplified examples to make it clearer.
Most religious people are Theist Gnostic, because they believe in a god and they're absolutely sure about it – that's why they believe in it.
People who say they're not religious are often Theist Agnostic, because they do believe in some kind of god-like thing, but they're certainly not sure and they don't follow a specific religion.
Atheists really don't believe there is anything god-like out there. But they're still usually Atheist Agnostic (and not Gnostic). This is because you can't actually prove that something doesn't exist. Of course, most Atheists believe you can disprove the gods portrayed by most religions, and in that sense they're Gnostic, but ultimately they usually agree that we can't truly know.