In contrast to science, scientism is not a method of observation or of inference but a belief that the scientific is the only legitimate source of knowledge. That is, scientism is a strictly scientific world view. Scientism places an overwhelming - some would say undue - focus on empirical methods to learn about the world. Other sources of knowledge, be they philosophical, intuitive, value-based or, God forbid, revelatory, are by definition worthless. Taken to its extreme, scientism itself becomes a kind of religion, in that its adherents accept scientific conclusions uncritically and refuse to even entertain arguments that do not have a strict empirical basis. It also tends towards arrogance, for example that of Carl Sagan calling evolution a "fact" and not a theory. At this point, scientism becomes dangerous because it jettisons the very safeguards that make science such a valuable method of acquiring knowledge. Certainty leads to the arrogance of ideological purity, which is destructive of authentic efforts to increase understanding.
So scientism, in its strongest sense, is the claim that only scientific claims have any value, but scientism is not itself a scientific claim, so if it is true, then it must be false. This, if nothing else, should serve as a poignant reminder to scientists that 1) we can't really be sure of what's true, only of what's not true, and 2) that a little philosophy can actually help to cast our empirical studies in clearer light.