The case is, traditionally, hung on the door post on the right, on an angle, tilted toward the door. The letter that looks like a "W" on the case, is the Hebrew letter "shin". It stands for the word "Shaddai", which is an epithet for God. As with almost all things Jewish, there are several theories on the derivation and meaning of "Shaddai". I think the one that makes the most sense in context of the mezuzah is that it is an acronym standing for "Guardian of the Doors of Israel" (Hebrew: שׁוֹמֶר דְלָתוֹת יִשְׂרָאֶל), which is commonly found as carvings or writings upon mezuzah cases.
The scroll, or the "mezuzah", is a parchment upon which is a portion of the Torah called "The Shema" ("Listen") and the second paragraph of the Shema, which is the "V'Ahavta" ("and you shall love"). The words are written by a Torah scribe, by hand, with no mistakes or corrections, in order for it to be a Kosher scroll.
When we leave our homes, and when we enter our homes, we touch the case and put our fingers to our lips to acknowledge that we are bound to act a certain way in the world and in our homes. We are reminded to teach God's law to our children and to be an example to others.
Below is one of my mezuzah cases with the parchment that gets rolled up and put inside the case and the translation of the of the Hebrew:
(Shema) Hear, O Israel, the Eternal is
our God, the Eternal is One.
Blessed be God’s Name and glorious kingdom forever and ever.
(V'Ahavta) You shall love the Lord your God with all your heart, with all your soul, and with all your might. And these words, which I [God] teach you this day, shall be upon your heart. You shall teach them diligently to your children, speak of them when you sit in your house, when you walk by the way, when you lie down and when you rise up. And you shall bind them as a sign upon your hand, and they shall be for a reminder before your eyes. And you shall write them on the doorposts of your house and upon your gates.