Agnosticism can be explained with a simple statement. I will borrow from renowned physicist and science-writer Carl Sagan for this, who said (in the context of searching for extraterrestrial intelligence), “extraordinary claims require extraordinary evidence.” He said this after stating that the “absence of evidence is not evidence of absence.” To say God exists is an extraordinary claim. It requires extraordinary evidence.
Following and seeking evidence is a recent phenomenon, because we now have the means to seek evidence. In ancient and medieval times, evidence was limited to what people perceived with their immediate senses. In those times, the best recourse was to resort to speculation and “generate” reason to believe in speculation. Modern science developed in a different trajectory, and sought evidence and detailed methodology in establishing speculation. Science developed from the speculating tendencies of ancient and medieval philosophers when Robert Boyle and others founded “experimental philosophy” in the 17th century. Experimental philosophy tried to establish the idea of knowledge based on evidence and “matter of fact”. Initially, “scientists” (or more correctly “experimental and natural philosophers”) were all God-fearing Church followers. They couldn’t attempt conceiving anything different, because science and empirically established facts were marginal and there were more facts left unexplained than facts with explanations. God and theology provided easy explanations to all those things that were inexplicable. Indeed, Boyle’s experimental philosophy developed under the tutelage of the Church. In fact, universities like Oxford were primarily meant to teach the clergy. During this period of time, it was impossible to defend atheism or agnosticism.
But as experimental philosophy evolved and “science” emerged, the reasons that sustained the belief of “God” were increasingly displaced. Darwin’s theory of evolution takes the most prominent place in this body of evidence that broke the back of theism. Now the accumulating evidence in favour of evolution from the fields of genetics and paleontology is so overwhelming that it has become an almost indisputable “fact”.
The confidence in the evidence-generating capacity of science is what consolidated agnosticism in a big way in modern times. That’s why more and more people embrace agnosticism in nations where science has evolved and is in an advanced state.
Science in its conception maintains a position of agnosticism. It implies that while the absence of evidence is not the evidence of absence (of something), you require extraordinary evidence to substantiate an extraordinary claim. The existence of God and fairies and demons are examples of some such claims.
Atheism, on the contrary, is a position that explicitly talks about the absence of something (a deity for example). It is a metaphysical position. Metaphysics comments on the “nature of things”, while agnosticism comments on the nature of knowledge. Agnosticism is an epistemological (epistemology is the theory of limitation and validity of knowledge) position or belief in the limited ways of knowing things, and the absence of alternative ways of knowing (refuting the claims of those who claim that they know more than the rest of the people – usually by making use of means not accessible to all). It does not comment on the nature of things, but on the nature of perceiving things.
While agnosticism is perfectly defensible, atheism is not. But strong agnosticism is almost the equivalent of atheism. The only difference being that it does not make atheistic statements explicitly. Instead, it suggests that the extraordinary nature of the claim of God cannot be believed with the absence of appropriate (extraordinary) evidence. New Atheism, propounded by the likes of Richard Dawkins and Sam Harris, however, alludes to the idea of “absence of evidence is the evidence of absence” in the context of God, because given the phenomenon attributed to God, it is very unlikely that it does not produce evidence of itself. This is certainly true of the Semitic religions: an omnipresent and powerful entity will likely leave footprints of themselves that can be validated using the routine tools of science. That this is not the case is the evidence that absence of evidence is the evidence of absence.