Yes, it's different from 'atheist'
We try other words: "humanist," "non-religious," "heathen," "infidel." Some of us even lie about the term "agnostic," and pretend that it means "atheist with an open mind" or "atheist lite," but it really doesn't.
—Penn Jillette, CNN.com, 2 June 2016
It really doesn't. Agnostic refers to someone who doesn’t know whether there is a god, or even if such a thing is knowable.
We've written about the origins of—and distinctions between—secular, atheist, and agnostic before. It's a subject that's sure to get more attention in the future; as both Jillette and the Pew Research Center have noted, “the percentage of Americans who are losing their religion is rising.”