On the doorposts of houses and rooms.
A mezuzah is a small box that is placed on the right doorpost of Jewish homes. Inside the box is a parchment scroll with verses from the Torah inscribed on it, including the Shema prayer (Deuteronomy 6:4-9, 11:13-21). The concept of a mezuzah comes from the Torah, where we read, “And you shall inscribe them on the doorposts (mezuzot) of your house and on your gates” (Deuteronomy 6:9, 11:20).
The box that holds the scroll can be decorated in a variety of ways, and often has a Hebrew letter shin on it, which is the first letter of one of God's names, Shaddai.
Some people believe that the mezuzah provides special protection over the residents of the home where it hangs, and when they pass by a mezuzah some people have the custom of kissing it (usually touching the mezuzah with their hand, then kissing their hand).