I shall start with a simple definition: a stereotype is an axiomatic assumption about people or things (note: landfills) that is usually false. False stereotypes are especially common when applied to a member of members of a group or place that the viewer has not met/seen/been to. Stereotypes are usually taken to be true due to jumped gaps in the facts, or (often illogical) things that the viewer assumes, such as that people they view as fat and ugly people must therefore also be stupid.
Stereotypes are also sometimes things that were once true (landfills are smelly and unsanitary) but are not true anymore. Other times, they may have an easily identifiable cause that makes many, although almost never all, cases fit the stereotype. Take the Dumb Blond stereotype. This stereotype applies to blond girls, and predicts that they will be rather dumb. Many blondes are usually considered to be cute in their early years of school. They learn that they can rely on their looks more than their brains, that they have the "luck" to be considered popular. They come to grips with the fact that they can even influence the teachers to get good grades, and so become regurgitators of information, spitting back what the teacher wants to hear and then forgetting it in a few (weeks|days|hours|minutes|seconds).
For one moment, let me indulge in my discursive nature:
In regards to both the "dumb blonde" stereotype, the current
school system does not
help much either. Almost no real
knowledge is required to get good grades; just commit the material to
short-term memory, look in the book to get the answers and write 'em down. Any moron can pass school, and in fact,many do.
Back to the topic at hand... A common stereotype I mentioned above is that "Landfills are dirty, smelly places that have rats crawling around in them and, should one appear next to my house in a spooky, occult fashion, would instantly make the property value of my house drop a hundred grand." This may have been true in the 1950's, but today, landfills are fairly high-tech and are very sanitary. The accumulated waste is packed down with steamrollers and then covered with dirt. The "bowl" that the waste is contained in is both gastight and watertight. Pipes carry away liquid waste and poisonous gases like methane.
Stereotypes are also used to simplify life. If you tried to keep a "database" of every single person that you met, recording such facts as the exact geography of their face, the various characteristics of their voice, their height, eye color, skin color, hair texture, body proportions, etc, etc, you would simply not be able to do it. However, a stereotype such as what a German's voice sounds like is much more useful compared to remembering the voice of every German that you meet. By making a rough estimate on such details, one is able to both learn and therefore utilize more information.