What do people mean when they refer to "Jews" in a racial context?

I never realized that Jews constituted a unified race of people. My great-grandfather was a Polish Jew. There are Israeli Jews, Russian Jews, Ethiopian Jews, and Jews from far east Asian countries. So why are they referred to as a race of people, rather than just a belief system? That would kind of be like saying that Christians are a racial group separate from white, black, Asian, Hispanic/Latino, and Islanders. I don't get it.