A mezuzah is affixed to the doorframe of Jewish homes to fulfill the mitzvah (Biblical commandment) to inscribe the words of the Shema "on the doorposts of your house."(Deuteronomy 6:9) Many families place a mezuzah on the front door only, but observant Jews fix one on every doorway in the home apart from bathrooms, and closets too small to qualify as rooms. 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 indelible black ink with a special quill pen. The parchment is then rolled up and placed inside the case.
Here are some photos in a slide show of the Rabbis in Guam putting up Mezuzos.