A mezuzah (Hebrew: מְזוּזָה‎ "doorpost"; plural: מְזוּזוֹת mezuzot) is a piece of parchment (often contained in a decorative case) inscribed with specified Hebrew verses from the Torah (Deuteronomy 6:4-9 and 11:13-21). These verses comprise the Jewish prayer "Shema Yisrael", beginning with the phrase: "Hear, O Israel, the יהוה our God, the יהוה is One" A mezuzah is affixed to the doorframe in Jewish homes to fulfill the mitzvah (Biblical commandment) to inscribe the words of the Shema "on the doorposts of your house" (Deuteronomy 6:9). Some interpret Jewish law to require a mezuzah on every doorway in the home apart from bathrooms and closets too small to qualify as rooms.[1] The parchment is prepared by a qualified scribe (a "sofer stam") who has undergone many years of meticulous training, and the verses are written in black indelible ink with a special quill pen. The parchment is then rolled up and placed inside the case.
This information is taken from Wikipedia -
http://en.wikipedia.org/wiki/Mezuzah