What is Aperture?
Aperture refers to the opening of a lens, through which light passes. It is setup in f/stops and is generally written as numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11 and 16.
Lower f/stops give more exposure because they represent the larger apertures, while the higher f/stops give less exposure because they represent smaller apertures. This may seem contradictory but will become clearer as you take pictures at varying f/stops.
Depth of Field
Depth of field is the area of acceptable sharpness in front of and behind the subject. Simply, how sharp or blurry is the area behind your subject.
The lower the f/stop—the larger the opening in the lens—the less depth of field—the blurrier the background.
The higher the f/stop—the smaller the opening in the lens—the greater the depth of field—the sharper the background.
Aperture and Shutter Speed
Using a low f/stop means more light is entering the lens and hence the shutter doesn't need to stay open as long to make a correct exposure. The opposite is true: using a high f/stop means that less light is entering the lens and therefore the shutter will need to stay open a little longer which translates into a slower shutter speed.