Secular humanism is much broader than atheism and entails many things that atheism does not. This will be easy to understand if you remember that atheism refers to nothing more than the lack of belief in any sort of gods. So what is secular humanism?
We begin with humanism. According to Paul Kurtz, "Humanism is an ethical, scientific, and philosophical outlook" which can be traced "back to the philosophers and poets of ancient Greece and Rome, Confucian China, and the Charvaka movement in classical India." Humanism is an optimistic stance, entailing "confidence in the power of human beings to solve their own problems and conquer uncharted frontiers." Humanists believe that reason, science, and technology can benefit humanity and seek to promote their growth.
Adding the "secular" modifier in front of humanism stresses scientific naturalism. That is, secular humanists are those who reject the existence spiritual/supernatural entities because there is no evidence for any such entities. in this way, secular humanists go much farther than atheists, rejecting not just the notion of god(s) but of anything supernatural. Secular humanists maintain that reason and empiricism are the paths to knowledge.
The secular humanist understands that religious faith has nothing to do with knowledge. It is not a different way of knowing because it is not a way of knowing at all - it has no bearing on knowledge whatsoever.
Ethically, secular humanism has something important to offer. Secular humanists believe that ethics are derived from reason rather than superstition. Some of the core ethical precepts (e.g., the "Golden Rule") predate Christianity. This poses no problem for secular humanists.
In the political realm, secular humanists are proponents of democracy, valuing human rights and their application to all humans. Thus, secular humanists are committed to humanity and less likely to be infected with the idiocy of patriotism. The humanist strives to promote human dignity and respect, discarding inherently divisive religious dogma.
This post is an abbreviated version of a previous post on Atheist Revolution titled "Secular Humanist First, Atheist Second."