The Mezuzah is a scroll (in a box) which is affixed to the every doorpost (save the bathroom) of most Jewish homes. The writing on a mezuzah is very small indeed, and the parchment is very fragile. As a result, aging shows easily on the parchment and can make it unkosher - unable to be read - and hence every mezuzah should be checked at least twice every seven years.
The Mezuzah contains extracts from the Torah:
Deuteronomy 6:4-9 (The Shema)
Deuteronomy 11:13-21
On the back of the parchment, which is rolled inside the tube/case, the Hebrew letters Shin, Dalet, Yud, which spell one of the names we use for G-d (Shddai).