What is the difference between Jehovah's Witnesses and Atheists? Don't they both NOT believe in God?
JW's believe in "a" god, and believe they are the only ones who are worshiping the correct God (the Lord in the Bible). Yet they misinterpret the Bible and choose to skip over the scripture/passages that don't agree with their beliefs.
So technically speaking, JWs are not Christians, for they separate themselves from Christians and recognize them/us as subjects for conversion to their (mis)understandings. But JWs are also not Atheists, for they do believe in "a" God. Some even claim Christ's name, but focus more on His teachings for good works than His doings and free gift of salvation. Here's a good link to help understand more about JWs.
Atheists, on the other hand, have CHOSEN not to believe in (a) God at all. It's not that they don't know, for those are "agnostics". And it's not that they're trying to find out, for those are "seekers". Atheists are people who CHOOSE not to know, while also rejecting and fighting everything that points to the possibility of God's existence. Here's a good link to help understand more about Atheism.