How does Aperture work?

The aperture is like the iris of the eye. When the aperture is very small, you will get a sharper focus and more depth of field-- near and far things in the scene will tend to be sharper. This is because the smaller circle is cutting down on the "confusion" caused by the countless overlapping circles of light being focused on the film or other light sensitive surface in the camera. The down side is that because the iris is smaller, less light is getting in, and you need a longer exposure to get a good image. When the aperture is larger, you can get a good image with a faster exposure, but sharpness and depth of field may suffer a little. The "circles of confusion" are larger, because the aperture is larger.