The intensity of the light reaching the sensor is controlled by varying the lens opening.
The lens opening is an iris-like mechanism in the lens.
Aperture, f/stop, and diaphragm, are synonyms for lens opening.
The numbers (which are logarithmic) used to designate aperture are confusing.
Use this mnemonic to help remember what the numbers represent.
The bigger the number, the less light reaches the sensor.
You can set the aperture by using the aperture-priority exposure mode.
In this exposure mode, you change the lens opening, and the camera selects the shutter speed.
On Nikon cameras, and many others, this mode is called A.
On Canon cameras, the mode is called Av.
If you're using the aperture-priority exposure mode, you'll be more aware of depth-of-field.
Depth-of-field is whether the background will be sharp or not.
We'll cover depth-of-field later.
Next, we'll look at how shutter speed controls the duration of the light striking the sensor.