Atheism is a Greek term which means essentially 'without [a] God'. Its meaning has changed over the centuries and has meant different things to different people. For some theists, it means 'actively believing there is no God' whereas for most atheists it means 'Not holding a theistic position, or not holding a belief either IN a God or that one actually exists'. Atheists would treat the concept of a god as an entirely human construct, with no meaningful attribute.
Most skeptics are atheists, but the reverse is not necessarily true. As most God claims must at some point fall back onto a faith position, there cannot therefore be sufficient evidence to conclude there is a God at all, therefore if a skeptic is doing it right, atheism is the only logical standpoint.
For ancient Greeks and Romans the term came to mean someone who did not worship the official Gods, so Jews, Christians etc would have been classed as atheists. In the past 20 years or so atheism has become a more popular position and though the term itself is still not widely accepted as a label by many who likely are one, in Western Europe at least, when used as atheists use it, it is probably the most common position held on the 'God question'.