Exposure is the amount of light captured by a camera's sensor. Capturing too much light is called over exposure. The resulting photograph would be very bright with little to no detail in the whites. The opposite of over exposure is called under exposure (too much detail lost) and its most extreme form would be a completely black photo.
In automatic mode, the camera is programmed to set the exposure for the photos at around 18% grey. This means a snow covered mountain will be grey and black velvet will look grey as well. This prevents a significant loss of detail in both the highlights and shadows. For example, exposure issues can occur outdoors in bright sunlight conditions, as the sky may be too bright. The camera needs to bring down the total brightness of the photo to compensate for the bright sky thereby making the faces darker. In manual mode, the lens aperture and shutter speed can be changed to obtain the required exposure. Many photographers choose to control aperture and shutter independently because opening up the aperture increases exposure, but also decreases the depth of field, and a slower shutter increases exposure but also increases the opportunity for motion blur.
Aperture refers to the size of the opening in which light enters your camera. The aperture is located on the lens and on manual and most old lenses, there is a ring on the lens that you can twist to set the aperture. It determines the light intensity of an image and they are listed in terms of f-numbers: the larger the aperture the lower the f-number. A wider aperture allows a faster shutter speed to be used for the same exposure. The higher the f-number (small aperture) the smaller the opening, so less light gets through and therefore a greater depth of field is achieved. Since aperture controls the amount of light being let into the lens, in lower light conditions, it is best to have settings that allow for the most light to be let into the cameras lens. It would be...