Exposure Triangle

The term Exposure Triangle contains the three main components that determine the exposure of a photograph: shutter speed, ISO, and aperture.  These three aspects directly affect one another and the Exposure Value (which is the sum of the three aspects' numerical values).

Shutter Speed

Shutter Speed is essentially just that - the speed of the shutter.  This is perhaps the most simple aspect of the exposure triangle.  The camera shutter is a curtain in front of the camera sensor that only opens when the camera takes a picture.  It opens to allow the sensor to collect light for the image and then it closes to stop light from hitting the sensor.  Long shutter speed is ideal for night photography or when creating a sense of motion like in a waterfall, however, this can also result in blurry images.  Fast shutter speeds are ideal for freezing action.  This affects exposure quite directly as the longer the shutter remains open, the more light is allowed into the image, resulting in a more exposed image.  

ISO

Another aspect of the exposure triangle is ISO, which is a camera setting that lightens or darkens a photo.  An increased ISO number results in a brighter image. Clearly this assists in low light photography.  However, raising ISO can make an image appear grainy so ISO should be used with caution. 

Aperture   

The hole within a lens, which allows light into the camera body, is referred to as aperture.  This affects the exposure (essentially, the brightness) of the photograph because it changes sizes to allow more or less light into the camera sensor.  It also affects the depth of field in the photograph - a large aperture equals a large amount of background blur and a small aperture results in a small amount of background blur.