Theism proclaims that there is a mysterious transcendent God out there. We need to know and understand this higher power. We can find a way to describe our transcendent God by using human terms. Most religions believe in some higher power. The place and how contact is made with this mysterious power is often different among theist believers. Theism is a positive response to the question of God, as opposed to the negative response of atheism.
Atheism proclaims that there is nothing out there, no transcendence, no God. Therefore, there is no theism or belief in God. There is nothing but us mere humans, so that we have to do the best that we can do. There is nothing but individual concerns that sometimes merge into some common causes. This world is full of nothingness or just plain old materialism. Get as much as you can now. There is nothing more. Quite often, this is a reasoned position, in that atheism is a minority opinion, since the natural tendency of most people is to believe in something other than yourself.