The Mezuzah signifies the sanctity and blessing of the Jewish home. Actually, it is a portion of the Holy Torah, hand written on parchment, in the same manner and script as the Torah. There are many laws and traditions that govern who can write this scroll and how it is to be written. It is this scroll which is called a "Mezuzah".
The Mezuzah may be inserted into any number of decorative "Mezuzah Covers". These may be decorated in many different ways and provide an opportunity to each person to choose a style that they like. But it is is the scroll which is crucial. It is an "original" sanctified document in accord with laws and precepts that dare not be compromised. On this sacred scroll of parchment are inscribed two passages from the Torah, "Shema Yisroel" and "Vehaya" (Deuteronomy 6:4-9 and 11:13-21).
"Hear, O Israel: The Lord our G-d, the Lord is one. Love the the Lord your G-d with all your heart and with all your soul and with all your strength. These commandments that I give you today are to be upon your hearts. Impress them on your children. Talk about them when you sit at home and when you walk along the road, when you lie down and when you get up. Tie them as symbols on your hands and bind them on your foreheads. Write them on the doorframes of your houses and on your gates."
"So if you faithfully obey the commands I am giving you today -to love the Lord your G-d and to serve him with all your heart and with all your soul - then I will send rain on your land in its season, both autumn and spring rains, so that you may gather in your grain, new wine and oil. I will provide grass in your fields for your cattle, and you will eat and be satisfied. Beware lest you be enticed to turn away and worship other gods and bow down to them. Then the Lord's anger will burn against you, and he will shut the heavens so that it will not rain and the ground will yield no produce, and you will soon perish from the good land the Lord is giving you. Fix these words of mine in your hearts and minds; tie them as symbols on your hands and bind them on your foreheads. Teach them to your children, talking about them when you sit at home and when you walk along the road, when you lie down and when you get up. Write them on the doorposts of your houses and on your gates, so that your days and the days of your children may be many in the land that the Lord swore to give your forefathers, as many as the days that the heavens are above the earth."
It is customary, upon entering or leaving a residence, to touch the Mezuzah with the index and middle fingers of the right hand. This reverence acknowledges our belief in "Shema Yisroel," the Jewish declaration of faith, which expresses the unity of G-d, the duty of loving, and serving Him with our whole being. It is our obligation to observe this mitzvah in and out of the home.
The presence of the Mezuzah on the right hand side of our doorpost represents basic principles of our faith. It is our responsibility to visitors as well as to ourselves!
When a Mezuzah is not valid, its blessings are nullified. Even if one letter or part of a letter is missing or imperfect, the Mezuzah may not be used. Only a "Kosher" Mezuzah sanctifies and protects the home, as well as fulfills the saying of the Psalmist, "G-d will guard your going and coming from now and for all time."
That is why when you purchase a Mezuzah for yourself or make it a gift to a relative or friend, you are obligated to assure its validity. Be certain that it is handwritten and that you obtain it from a reliable source.