Humanism is a term that has been used in many different contexts throughout the years, and that can mean many different things. There are as many different interpretations on what humanism can be as there are interpretations of what human nature is. Many different schools of thought have been humanistic without overtly claiming to be so, while many types of supposedly humanistic philosophy have perhaps not been humanistic at all.
Currently in the modern western world, humanism is synonymous with secular humanism, the belief that human progress does not need transcendent theology to work. However, this has not always been what humanism meant.
The Ancient Greeks had a system of humanistic thought, although it was perhaps not the high point of their culture. While with its practicality and appreciation for human form and human life, the Greeks could perhaps be argued (especially amongst proponents of Western Civilization) to be the most humanistic of ancient civilizations. However, the Greeks had a definite metaphysical and speculative bent that could be seen to be against humanism. And while it was a greek thinker, Protagoras who said "Man is the measure of all things", he was widely regarded as a sophist.
The Classical Chinese developed a type of humanistic thought in the warring states period that would last for (so far) another 2500 years. Confucius, and later Mencius, would annunciate a philosophy that took it for granted that ren, the ability to understand and act humanely, was the starting point and goal of human action and thought. The full history and scope of Confucianism is beyond this node (as well as defenses against those who think Confucianism is simple a fancy word for authoritarianism , but it is obvious that Confucianism has always been a humanistic school, and often in the best sense of the word.
The word humanism itself comes to us from The Renaissance, and in many ways reflects nothing but a curriculum change in education at the time. It originally meant the Studia Humanitias, meaning a learning of rhetoric and history, rather then the abstract study of Aristotle, metaphysic, theology and so on. It also focused on grammar and language for the purposes of persuading people and civic life, rather then language based on supposed connection to metaphysical categories. This change in thought was associated with a great deal of the movements of the Renaissance, including art and banking and exploration and invention. However, renaissance humanists never were thinking that "man is the measure of all things", and for the most part, they still accepted the Catholic Church's hegemony and worldview.
Humanism as we have it now, in the form of secular humanism, is a mishmash of every enlightenment and liberation movement of the past 200 or 300 years, with a great deal of utilitarianism and what not thrown in. Humanism now is equal mixture of Immanuel Kant, Thomas Jefferson, Martin Luther King, Jeremy Bentham, Karl Marx, Jean Paul Sartre and Victor Hugo. In other words, a philosophy that means so much that it means nothing at all. In addition, I think that secular humanism, since it takes scientific progress through scientific reductionism to be a great good, it is not actually all that humane. While science is indeed rational, it is still a system of thought that relies more on Aristotle and his tradition then it does on natural human experience. Of course, there has also been a recent anti-scientific humanist movement. Which, of course, goes to show that humanism can mean anything possible, as long as it somehow has to do with human nature, human potential or human values.