The Mezuzah and the Klaf

Many Jews, and now even Christians, will affix a mezuzah to the doorpost of their homes to fulfill the mitzvah to “write the words of God on the gates and doorposts of your house” (Deuteronomy 6:9, 11:20). Some will put a mezuzah in every doorway in their home, except closets, laundry rooms, and bathrooms, etc. as these are not living spaces. But what is a mezuzah?