One lexicon defines a humanist as someone who is concerned with the interests and welfare of humans. But the matter is more complex. I found a very interesting 10 point definition on the web:
1) Humanism believes in a naturalistic metaphysics or attitude toward the universe that considers all forms of the supernatural as myth; and that regards Nature as the totality of being and as a constantly changing system of matter and energy which exists independently of any mind or consciousness.
2) Humanism, drawing especially upon the laws and facts of science, believes that we human beings are an evolutionary product of the Nature of which we are a part; that the mind is indivisibly conjoined with the functioning of the brain; and that as an inseparable unity of body and personality we can have no conscious survival after death.
3) Humanism, having its ultimate faith in humankind, believes that human beings possess the power or potentiality of solving their own problems, through reliance primarily upon reason and scientific method applied with courage and vision.
4) Humanism, in opposition to all theories of universal determinism, fatalism, or predestination, believes that human beings, while conditioned by the past, possess genuine freedom of creative choice and action, and are, within certain objective limits, the shapers of their own destiny.
5) Humanism believes in an ethics or morality that grounds all human values in this-earthly experiences and relationships and that holds as its highest goal the this-worldly happiness, freedom, and progress’ economic, cultural, and ethical of all humankind, irrespective of nation, race, or religion.
6) Humanism believes that the individual attains the good life by harmoniously combining personal satisfactions and continuous self-development with significant work and other activities that contribute to the welfare of the community.
7) Humanism believes in the widest possible development of art and the awareness of beauty, including the appreciation of Nature’s loveliness and splendor, so that the aesthetic experience may become a pervasive reality in the lives of all people.
8) Humanism believes in a far-reaching social program that stands for the establishment throughout the world of democracy, peace, and a high standard of living on the foundations of a flourishing economic order, both national and international.
9) Humanism believes in the complete social implementation of reason and scientific method; and thereby in democratic procedures, and parliamentary government, with full freedom of expression and civil liberties, throughout all areas of economic, political, and cultural life.
10) Humanism, in accordance with the scientific method, believes in the unending questioning of basic assumptions and convictions, including its own. Humanism is not a new dogma, but is a developing philosophy ever open to experimental testing, newly discovered facts, and more rigorous reasoning.
http://www.ihumanism.org/intro/10points.html
I think atheism is a proper subset of this (mathematically speaking), while traditional forms of theism and humanism share at least points number 5 – 7 and more modern forms of theism also share points number 8 – 10 and parts of points 2 – 4 as well, especially the role of science. For example that human beings are an evolutionary product of the Nature based on natural laws. The difference is only related to the explanation of the natural laws (which are either seen as being capable of explaining themselves or not).