Since I’ve been throwing the terms around a bit lately, I thought I’d make the distinction.
A belief or doctrine that inherent hierarchical differences among the various human races determine cultural or individual achievement, usually involving the idea that one’s own race is superior.
A belief in the existence and significance of racial differences, but not necessarily that any hierarchy between the races exists. Racialists typically reject claims of racial superiority.
Another way to put it is to say that a racist believes that interracial relationships are fundamentally wrong. A racialist, however, may simply prefer to date people within his or her own race because they believe other races will be incompatible.
The reason I make this distinction is that I believe racism is currently on the fringes of social thought, while racialism remains prevalent in the mainstream. What we call institutionalized racism would more accurately be described as institutionalized racialism.
At any rate, both are dangerous–to varying degrees–and must be combated with information, temperance, and understanding. 🙂