This is an excerpt from a post by Razib Khan on the topic (emphasis in the original):
Can 23andMe Tell Us If Jews Are A Race — And Is That A Good Thing? The author interviews scientists who know the science, but he still manages to garble and confuse everything. First, Ashkenazi Jews descend from a endogamous community which flourished in Central Europe probably no earlier than ~1000 AD. That is why a Ashkenazi Jewish cluster emerges naturally out of the population genetic data; there’s a real coherent demographic history being reflected. Whether that’s a “race” or not I’ll leave to the reader.
Second, the story states that “Sephardic Jews are not considered a distinct population by either company, or by researchers — their genetic make-up is not sufficiently different from surrounding North African, Iberian and Greek populations.” This very misleading. To a great extent Sephardic Jews are rather distinct from the surrounding populations. There is some evidence of shared ancestry in Moroccan Jews with Moroccan Berbers (I know because I’ve looked at a lot of this data), but it’s a small proportion. Similar things can be said about most Sephardic communities. But, they are not nearly as coherent a genetic cluster as Ashkenazi Jews. There has been some gene flow and assimilation with many local Jewish populations (e.g., the Syrian Sephardic Jews absorbed a local Levantine Jewish community, which had its own liturgy until the 19th century).
For what it is worth, the source article is pretty much useless unless you are more interested in why people care about their personal genetic ancestry and how they feel about it, then about the population genetic history of the Jewish community.