An agnostic, thus, in a way, does not completely trash the idea of the existence of a god or a ‘higher power’, but simply says that a search for the same is a futile exercise, one that will yield no results whatsoever. They say it’s very much like finding the absolute basic building block of all matter. Science has allowed us to advance from the notion of atoms being the most fundamental particles of matter, to giving us proof of the existence of much smaller, even more fundamental particles like quarks, leptons etc. But can these particles be made up of something still more fundamental? If yes, then when will it end? This is exactly what the agnostic has to say- we may never know or reach to the absolute base.
An atheist on the other hand disbands the very notion of god. He does not believe in the existence of a higher power, plain and simple. Often, such people will say they reject the idea of god as perceived by people in general. An atheist can thus, in a way, be said to harbour a more hard-line view on the entire dialogue.
An important point to note is that both types of views might have something in common- a lack of credible evidence supporting the presence of a higher power.