What are the differences between atheism humanism and secular humanism?

Humanists believe that the way in which one lives one's life should be dictated by common human principles and reasoning, and that one's actions should be directed toward the betterment of mankind, independent of any religion.

Atheists believe that there is no God, and therefore (by extension) that how one lives one's life must be dictated by secular (or non-theistic spiritual) principles instead of rules handed down by some deity. Atheists are often humanists of some form, but do not necessarily have to be (many sociopaths, for example, are atheists but are also definitely not humanists)

Agnostics do not claim to know whether God does or does not exist: there may be some deity or there may not be. Humanist agnostics go one step further to claim that they do not know whether God does or does not exist, but the question also should not matter in how one lives one's life (that it should be lived by humanist principles in either case).

So in summary, an atheist believes there is no God (and believes that is an important distinction to make). A humanist agnostic believes he/she doesn't know whether there is, but that it shouldn't matter in any case.