Atheism is the belief that there is no God. Secular Humanism does not necessarily believe that there is no God. The atheist would reject the idea that there is God. He believes that all of the things that are happening and all of the decisions that he makes is based entirely on his own experiences. Secular Humanism is not a belief but rather, a view of the world as a whole.
Someone who has this belief will not reject the idea that there is God because he may believe that God is needed for people to stay moral. Secular Humanism aims to explore the nature of morality more than religion.