Camera Modes

Most cameras have automatic, or semi-automatic shooting modes. In manual mode you control everything. Automatic modes can vary from camera to camera and manufacturer to manufacturer. Your camera may have a couple other settings it will change in the following modes, but these are the essentials that impact exposure.

In the following basic automatic modes the camera automatically determines:

• Full Auto: aperture, shutter speed, ISO, white balance, flash.
• Flash Off Auto: aperture, shutter speed, ISO, white balance.
• Program: aperture, shutter speed.
• Aperture Priority: shutter speed.
• Shutter Speed Priority: aperture.

Most cameras also have the following modes as well, their purposes are pretty clear:

• Movie
• Close Up
• Landscape
• Night Portrait
• Portrait
• Sports

Your purposes and skill with the camera will determine which mode you use. I recommend using Manual at all times, at least at first, so you can get a better understanding of what is affecting your exposure. And what to change if it isn't to your liking. The Shutter and Aperture Priority can speed things up at times without throwing in too many variables as well.

Exposure

A digital camera composes an image file by using a sensor, which then gathers that data to compose the image. The more light that the sensor is exposed to, the brighter, or more exposed the image is. The amount of light that is allowed into the camera can be changed by:

• The amount of time the shutter is open, which is the shutter speed. 
• The size of the opening of the opening which is the aperture also called the f-stop. 
• The sensitivity of the sensor which is called ISO. 

Impact on Exposure
Shutter Speed
A shutter speed of 1/125 is faster than 1/60. At faster shutter speeds there will be less motion blur because the amount of time the image is taken in is less. But the image will be darker at a faster shutter speed because there is less time for the light to get to the sensor.

Aperture
An opening of f10 is smaller than f8. A smaller opening makes the image darker because there is less room for the light to get in. But it allows for more in the image to be sharper (depth of field).

ISO
A setting of 100 ISO is less sensitive to light than a setting of ISO 1000. The more sensitive the ISO, the brighter the image. But the higher the ISO the grainier the image.

Therefore the following exposures are the same, but have different effects on the image:
1/60, f8, ISO400: Possibility of blur, more in focus, slightly grainier.
1/250, f2.8, ISO200: Low possibility of blur, less in focus, less grainy.