Aperture

Ok Mr. Reeder, I'm finally writing about aperture.

Simply, aperture is a small hole through which light is admitted. This hole can get bigger or smaller depending on how exposed you want the film to be.
A part of the lens that causes aperture size is called the diaphragm. The diaphragm works a lot like the iris in the eye. The diaphragm controls how wide the aperture is.
Oddly to some, reducing the aperture size does not reduce the depth of field (the amount of the photo that is focused). It actually increases the depth of field, bringing more in focus. So if one needs to take a picture with as much of the photo in focus as possible, they would want to decrease the aperture size.