Windows are important. They fit in holes in the walls, so that there isn't a draft. They also allow us to see the outside world, where someone may be stealing your brand-new, incredibly expensive car.
Why are windows made of glass? Well, basically because glass is clear and reasonably sturdy stuff. Windows are typically made clear so that you are able to see whether some guy is in fact stealing your car. Of course, you may ask, why were windows made clear before cars were invented to be stolen? Frankly, it's a mystery. Research is being conducted at several prestigious universities. Another important quality of glass is that it is much more solid than, say, water. If windows were made of water they just wouldn't work right.
An unfortunate side-effect of the clearness of windows is the fact that the guy stealing your car can also see whether or not you are sitting in your office, watching him steal your car and getting a description of him. This is why many criminals carry guns. This is also why many offices are now equipped with multiple turret-mounted machine guns. But that's another story. Anyway, this side-effect can be fixed in several ways, most commonly by covering the interior of the window at times when the window is really superfluous or unwished for. Methods include drapes, shutters, and venetian blinds. (The people in Venice need blinds because they use water for their windows. This also why they have canals instead of streets. I warned you that water just doesn't work right when used as a window.) Another important method is one-sided glass. This is really neat, because you can only see through it one way. No one is sure how this works either.
A popular style of window used for religious or decorative purposes is stained glass, which combines pieces of colored glass in such a way as to produce a pattern or picture. When the light shines through the glass, a pleasant (some say) image occurs. Unfortunately, stained glass can rarely be seen through. This is why people who go to church are poor -- they can't see people stealing their cars, and thus are forced ever lower on the economic scale.
Windows are also significant when broken. Often children are responsible for this. Occasionally, however, some irresponsible pilot breaks a certain thing called the sound barrier, causing another thing called a sonic boom, which can shatter windows. (This is significant, but will not be discussed here.) Broken glass contains sharp edges and can be quite painful if jabbed into any part of the body. Do not try this unless you are really stupid (I still wouldn't recommend it, even if you are stupid).
I find windows to be quite attractive and all together more useful than quite a few other things, like chickens.