Why is Judaism considered a race?

I hear people say things like "I'm half Jewish" and things like that. I thought Judaism was a religion, you know, being Jewish. How can you be half of a religion? Sure there are stereotypes like that they have big noses and there are a few more, but Jews don't come from the same ancestors. For instance, Ashkenazi Jews and Sephardic Jews are both "Jewish." However, whereas Ashkenazi Jews often hail from Europe, Sephardic Jews often hail from the Middle East. So what the heck? Why is it considered a race. Catholicism, Christianity, and many other religions aren't considered a race. So why is it considered a race?