Secular humanism differs from atheism and agnosticism in a way that is easy to state: it has documents. Where the other two are answers to a question, humanism is a set of positive commitments, written down, argued over, revised, and adopted by organisations with members.
The main texts are a short list. A Humanist Manifesto (1933) set out a humanism explicitly described as religious, with God removed but the vocabulary of religion retained. Humanist Manifesto II (1973), drafted by Paul Kurtz and Edwin H. Wilson and signed by figures including Andrei Sakharov, B. F. Skinner and Isaac Asimov, dropped that framing and was written in the shadow of the century's two totalitarianisms. A Secular Humanist Declaration followed in 1980, and Humanism and Its Aspirations in 2003. Internationally the reference text is the Amsterdam Declaration, first adopted at the founding congress of the world federation in 1952 and revised since.
What the documents share is a short and fairly stable core. There is one world and no supernatural agency in it. Ethics is a human undertaking, answerable to human and animal welfare, worked out by reason and experience and therefore revisable rather than revealed. Inquiry must not be fenced off by dogma. The state should be neutral among beliefs and should not privilege any of them, including this one. Meaning, purpose and beauty are to be found within a single finite life, which is the only one anyone gets.
Two internal tensions are worth naming. The first is whether humanism is primarily a fighting secularism, occupied with church and state, or primarily a life stance offering people something to live by; organisations lean different ways. The second is the older split between religious humanism, which keeps congregational forms, and secular humanism, which rejects them. Ethical Culture societies still sit on one side of that line and most national humanist bodies on the other.