Unfortunately, like many sensitive subjects in modern culture, there’s not a lot of agreement about what either term means, it seems to me. I’ve rarely found people taking the time to even consider the kinds of distinctions made in @SavoirFaire‘s and @jerv‘s answers above.
I appreciate the detail of @SavoirFaire‘s answer for the purpose of philosophy, but it’s pretty complex and doesn’t match what I have long thought everyone generally (sloppily) meant by atheist and agnostic.
It also seems weird from one of my own favorite ways to get sophisticated about religion, which is to notice that it seems to me that all the theologians I know about whom I respect/understand the most, seem pretty clearly to all be relating to their various religions in metaphorical and religious ways. If I combine this with “atheism” as being disbelief in literal gods, then it seems to me that some Christian theologians and even clergy would technically maybe be considered atheists, which seems quite wrong, and I don’t know if that would be because they are two different things, or what. Maybe it’s not that they’re atheists, but that what a God is is not literally a person, but a symbol for the universe or some aspects of it, and of course they do believe in it and understand how it’s used as a metaphor. Which brings me to think that there’s not really that much of a cosmological issue about believing in God or not, between such scholarly Christians and atheists, because they’re not talking about the same things, or rather, they are talking about the same things and using different words, and the Christian has an understanding for the metaphorical messages in Christianity, and values them, while the atheist probably doesn’t understand that or at least doesn’t know the details.