Usually when someone thinks of an atheist they conceive of only one facet of the position. An atheist doesn’t believe in God/gods. Full stop. But that position is limiting. It doesn’t say anything about living, about what is valuable, etc. And not all atheists are atheists for the same reasons. There are many ways to be an atheist, or to hold non-theistic beliefs. I’ll touch on three broad categories I attribute to one of my philosophy TAs from college. The categories are metaphysically atheistic, morally atheistic, and existentially atheistic.
The category that most atheists seem to fall into (or are perceived to fall into) is metaphysical atheism that is they will unequivocally say that God/gods do not exist. There is no reality above the physical reality under the domain of the sciences (there may be qualifications, but this is the broad gloss). This kind of atheism is a denial of the supernatural reality of theistic views. Think of this as a sort of first step, basic atheism.
Being morally atheistic may seem ambiguous, but here is how I think of it. Another common argument is that without God there is no morality. It is an argument commonly misattributed to Dostoyevsky. If there is no God, then everything is permitted (this is actually Ivan Karamozov, a character in The Brothers Karamozov). To be morally atheistic is to believe that God is not required for morality. One can be moral and ethical without a theistic worldview. There are many philosophers who have written on this subject, starting with Aristotle and continuing through Singer, and still more. Naturalistic ethics is harder to ground, but many atheists have deeply moral worldviews that have nothing to do with a supernatural reality. Sometimes this is the reason they leave their churches in the first place.
The last category I will outline is existential atheism. As I understand this, to be existentially atheistic is to find no value in the existence of God. Even if God does exist, it wouldn’t change how I construe value and meaning in life. Value in life can come from a variety of places on this view. For many it stems from interaction with friends and family, experiences in the world, their work, etc.
In this way it is not so simple to just be an atheist. For example, I am a metaphysical agnostic. I am perfectly willing to accept that God might exist. Based on the evidence, inferences can go one-way or the other. Morally I am an atheist. I believe that one can lead a perfectly moral life without being a Christian. Existentially, I am an atheist as well. Regardless of the existence of God I don’t believe that God, if it exists, is worthy of worship.
As I have said before, atheism in the classical sense is limited. I’ve mentioned secular humanism many times, but I find it to be an answer to the moral and value questions that follow from leaving a religious organization. Christianity, Islam, Judaism, they all have answers to what we should find valuable and what our moral worldview should be. Non-theists should have answers to those questions as well, because it is with regards to those questions that many of us left the churches, mosques, and synagogues in the first place.