A jalousie window is a window which is made with a set of movable glass slats, sort of like oversized built-in blinds. When the window is closed, the slats rest against each other, and when the window is opened, the slats separate, permitting plenty of fresh air into the structure. These windows were once extremely common in the American South, when houses could become insufferably hot before the advent of air conditioning.
The earliest reference to jalousies dates back to around the 16th century, and the basic design hasn't changed a lot since, although early jalousie windows were probably made with wooden slats instead of glass ones. The windows are typically manipulated with a crank which moves the housing on either end of the slats, causing them to open up or move down.
There are a number of styles of jalousie window. Some are made with clear glass, permitting plenty of light and allowing people to easily see in and out of the window. Others are made with frosted or tinted glass for privacy reasons, creating a more obscured view, and some jalousie windows are also protected with shutters, in case of hurricanes.
There is a big disadvantage to the jalousie window: these windows are basically impossible to seal. Inevitably, air will slip in between the slats, making the area around a jalousie window quite drafty. In climates which are warm year round, like the tropics, this isn't a problem, but in an area where the winters get very cold, a jalousie window can be a serious liability, because these windows are not very energy efficient.
In addition to causing heat loss, jalousies can also create a security risk. In most cases, it is very easy to remove individual slats of glass, creating an easy opening into a structure. For this reason, some regional building codes ban the installation of jalousie windows, encouraging people to use more secure and efficient windows to light and ventilate their homes.
Since air conditioning has become so widespread, the jalousie window has seemed less appealing to some home designers, although these windows do still crop up on occasion. If you happen to live in a house with jalousies and you are trying to figure out how to clean them, the best way to do it is to take the panes out and wash them individually, although you can also open the window and clean the slats with a large sponge, if you have hands small enough to fit in the gaps between slats.