Aperture
Aperture controls light intensity.
At the very rear of a lens are a series of fan-blades and the position of the blades (referred to as a lens Aperture's "f/stop" value), controls the strength of the light we are allowing to pass through the lens - much like a throttle controls the amount of gas getting to an engine.
A low f/stop value (say, f/2.8 or less) means as much light as possible is passing through the lens. This is great for low-light conditions or sports-action events.
At the other end of the spectrum (say at a value of f/22), the light is being severely limited, and hardly any of it is getting through. This is often preferable for very bright conditions or long-exposures.
Aperture f/5.6
Now here's the fun part.
A side-effect to an Aperture's value, is that it allows us, under the right circumstances, to control the area of focus in our photograph, something referred to as the "depth-of-field."
Here, I was photographing a granite slab, at a distance of 10-15 feet.
At the lower-end of the f/stop scale (with the blades wide-open and maximum light-strength), my subject is in focus but the background is blurry.
This is referred to as having a very limited depth-of-field, and is very common for portraits. Hence the "portrait" mode setting on a camera.
Increasing the f/stop value closes down these blades, resulting in much less light passing through the lens. At the same time though, this action also opens up our focusing range considerably.
Having pretty much everything in focus is very common for landscapes, hence a camera's "landscape" mode feature which will set a high Aperture value for us.
In this example, we see how the Aperture value manipulates the depth-of-field, from being very limited so our eyes are drawn solely to the rock, to where the background becomes clearly visible and a central player in our final photograph.
