The difference between an Atheist and an Agnostic is subtle, and generally relates to a difference in philosophy, not aim. Agnostics and atheists share the same aims, to a degree, and the same creed of progress by proper application of rationalistic principles. Both atheists and agnostics share an inherent distrust of any creed or principle derived from faith or personal revelation. This is why both groups tend to be skeptics by method, and are usually grouped under the same term of "infidels" by believers in the irrational. Both atheists and agnostics are (or should be) committed to a program of public education, in order to stamp out the rule of the irrational that has so long bedeviled mankind.
The only real difference is this: whereas an atheist is positive that no god exists, nor has at any time, an agnostic prefers to leave the question open, except to note that belief in a Deity has never been shown to be of any meaningful use to society. Quite the opposite, in fact. More than a few bloody wars have been fought over religious principles, and uncritical thinking still dominates the minds of the majority of humankind. An agnostic will also note that history reveals no Divine Guidance or Plan. On the contrary, what we learn from history is the inexorable succession of effect after cause, of consequence after choice.
On the matter of whether or not a god (or gods) exist, the agnostic prefers to remain silent. It can be shown that the Judeo-Christian god has a mundane history, a biography if you will. In general, humankind creates gods and devils in their own image. This, however, still does not preclude the existence of a silent Deity, or of an unconscious force that works to create order from chaos. These are questions that cannot, as yet, be answered. The agnostic simply prefers not to take a stand until all the evidence is in, something which may not happen for a very long time, and something which, in fact, may not happen at all.
On the matter of the human soul, the agnostic generally agrees with the atheist that no such thing exists. The marriage of modern neuroscience with cognitive psychology has revealed that the incorporate entity, which we call "mind", can be explained as the workings of an incredibly complex organ, the human brain. This organ contains hundreds of billions of neurons, with literally thousands of billions of connections between them. This incredible mass of wiring gives rise to consciousness. At the end of the day, everything that makes us human, our mind, our emotions, our judgment, our perception of reality, all are generated by the conscious organ that each of us carries in our heads.
Both the atheist and the agnostic are of the opinion that ethics and morality derive from humanity, not divinity. All systems of ethics can be viewed as necessary attempts to regulate human interaction. The pangs of conscience that believers attribute to Deity can be viewed as nothing more than the inevitable result of overstepping the bounds of our conditioning. This last neatly explains why morality, to some degree, is a plastic notion. That which some find abhorrent, others revel in. One man's sin is another man's pleasure.
Finally, atheists and agnostics are usually grouped together under the label "freethinkers". This group, which includes other viewpoints such as Secular Humanism and Rationalism, is united by a number of common aims and creeds, chief among them the notion that all truth must be derived from, and tested by, the principles of rationality.