While the shutter controls the amount of time that light can enter the camera, the lens's aperture dictates the size of the opening in the lens that light passes through. The aperture consists of a spiral of solid, over-lapping leaflets that contract or expand to adjust the area of the opening. In terms more easily understood, if the camera is a model of the human eye and the shutter operates like an eyelid, then the aperture fulfills the same function as a pupil.
The settings for the aperture are scaled in terms of f-stops, which is the technical term that denotes the diameter of aperture's opening. Each lens has an aperture ring around the base with a series of f-numbers imprinted on it that can be shifted to adjust the aperture's setting. If a photographer should prefer to not adjust the aperture manually, modern cameras possess the technology to toggle this setting electronically through the camera.
The trickiest part to understanding the aperture is remembering how the f-stop number corresponds to the size of the aperture opening. In essence, the smaller the f-number, the larger the opening in the lens; the larger the f-number, the smaller the opening in the lens. For example, a setting of f/1.2 is a very large opening for the aperture, while a setting of f/16 is significantly smaller. Perhaps the easiest way to remember this seemingly counterintuitive scale is to know that, for all things concerning the aperture opening, "less" is "more."
Once you've grasped how the aperture works, this will make it easier to understand its effects. By manipulating the aperture settings, you can adjust the sharpness of details in a photo as well as control the depth of field. By using a smaller f-number, you can increase how fine the details appear as well as reduce the depth of field in the picture. However, if you wish to keep items other than your subject in focus (such as background objects), a larger f-number is required. Additionally, if you are photographing a subject that is in motion and wish to minimize the blurriness in the photo, use the largest f-number possible. In doing so, you will increase the area of the aperture's opening and reduce the amount of time that the shutter needs to stay open, thus eliminating the blurriness.