Mezuzah actually refers not to the outer case but what goes inside: a small parchment scroll on which a certified scribe (sofer stam) has copied, in Hebrew, two passages from the Torah: Deuteronomy 6:4-9 and 11:13-21.
It is affixed to the right doorpost of the entrance to inhabitable rooms of Jewish homes.
Here is one of the two mezuzot in my home, this a simple one made of olive wood.
Here is the other one below. I opened the parchment for you to see the Shema, the main prayer recited daily, which is written on it. Click on the photo to see it enlarged.
The parchment is then rolled up like a scroll and the case is closed and nailed to the doorpost.