Agnostic when Thomas Huxley first coined it in 1869 meant according to him, that we cannot know anything beyond material phenomena as explained by Science, so far from a wooly-headed point of view , when it started it was a rationalistic view.
In modern time many label themselves Agnostic without appreciating this original meaning, and if you question them their postion boils down to "it hurts my head to think about it, so I use the label as a cop out".
Atheist, a word going back to the 16th Century, meaning without God was very similar, not saying there was no God, but that we could not know anything beyond the space-time continuum we live in.
The meanings ascribed to the two words today depend upon the context and how the person applies them to himself or what he/she thinks they mean, hence Cofty's wise attitude of lets not faff about with semantics.
New Atheist was coined to explain the antics of Dawkins and others who felt simple non-belief was not enough, that the religious beliefs of others should be attacked, an idea I go along with to a certain degree, young earth creationists for example deserve to be exposed as twats.
As people today have certain ideas in their head if you use either word about yourself I tend to style myself a secular humanist or something similar, if the person who needs the label wants to take things further I will explain my stance on God , Life, the Universe and even the virtues of real ale if they will listen.