Exposure is the combination of four element related to light: light, sensitivity, intensity and time. Unless you're using a flash or providing a light source, assume that light is a constant (i.e. you work with what the environment provides).
Sensitivity of film/sensor is measure by the ISO (international standards organization) rating. The higher the ISO, the less light is needed since the sensor is so... sensitive. Thus, in low light situations, one can use an ISO 3200 film and still use a high shutter speed to snap shots. However, the downside of a high ISO is that the result will have more noise (grainy) then a less sensitive (lower ISO) sensor/film.
Shutter speed is the time the curtain of the camera remains open when a photo is taken. It is measured relative to one second so a shutter speed of 25 means that the curtain is open for 1/25th of one second. A fast shutter speed means less time for light to pass through to the sensor but allows for sharper pictures. With a long shutter speed, there is a higher chance that the photo will be blurred since during the time the curtain is opened the subject could move or your hand wasn't steady.
Aperture is the size of the lens opening that controls the amount of light that reaches the sensor. Lens aperture is specified in f-stops and the higher the f-stop the less light is allowed to pass through the opening. At first I was always confused by aperture terminology and didn't really understand how the number is derived.
The aperture of the lens effects the amount of light passed through and that in turn effects the depth-of-field. The equation for determining aperture:
When comparing lenses, the focal length (f) can be set as a constant of 1 thus the higher the f-number (N) the smaller the opening and thus allows less light.
The combination of these different factors equals the exposure of a photograph. As you can see, the same exposure can be obtained with different combination.
