The mezuzah is a very important and visible symbol of a Jewish building or room. It establishes that a space is in some way tied to Jews–by use or ownership.
Mezuzah is a biblical Hebrew word for doorpost but usually refers to the ritual and/or decorative item that is placed on a doorpost. See dictionary.com for examples of pronunciation.
It is often a small oblong item which contains a handmade parchment scroll written by a trained scribe with verses from the Torah (sometimes referred to as the Five Books of Moses). These verses, part of the Shema prayer, are passages from Deuteronomy (6:4-9 and 11:13-21). The tradition stems from verse 11:20: “And you shall inscribe them on the doorposts (mezuzot) of our house and on your gates.”
By marking the liminal space of a doorway, a Jew both highlights their personal Jewish identity and demonstrates that this identity may be different in public versus private. Jews do so for many reasons including: out of a sense of tradition (that is what Jews do), based on centuries-old beliefs that mezuzah provides protection, to remind ourselves of entering and exiting home (which is the context of the Biblical verse above), and out of a expression of Jewish identity.
While the insides of a mezuzah are fairly similar (containing the parchment above or a reproduction of one), the case can vary as an opportunity for the owner to express style and personality. Many have a Hebrew letter (ש, shin), the first letter of Shadai, one of the names for God in Judaism. A google image search can show the range of styles that a mezuzah may take.
The size and shape of a mezuzah is determined by factors including the size of the doorpost and the size of the parchment (parchment is expensive so price typically goes up with size). They vary from under two inches and the size of a pencil to extending several feet for ceremonial versions in significant Jewish buildings. Most are a few inches long and up to an inch wide. The price of a mezuzah case depends on material, size, and if it is handmade (especially by an artist). They are typically placed on the opposite side of the doorway from the door hinges at the mezuzah is primarily found in doorways, some Jews have a mezuzah in their car (usually hanging from the rear view mirror) or in the form of a necklace. If space is available the mezuzah typically is angled in to the room or building.
Some Jews kiss the mezuzah whenever they enter a door that has one (touching it with a finger or hand and then kissing it) to add a physical action to the mental act of paying attention to the mezuzah as they enter it.