Simple things to remember about exposure

Exposure and the camera's meter:

A photograph is correctly exposed when it looks like you intended, not brighter or darker. 

underexposed picture

correctly exposed picture

overexposed picture

The camera's light meter will show 

• "0" when the exposure is "correct";
• Positive numbers when it is on the bright side; 
• Negative numbers when it is on the dark side.

Aperture:

• Depth of field determines how much of the photo is in focus;
•  Aperture adjusts how large of an opening the lens makes to let light in;
•  Small aperture numbers like f/1.8 correspond to a larger opening. This lets more light in and depth of field will be shallow;
• Big numbers like f/18 correspond to a smaller opening, less light in, and deeper depth of field.

sarge
aperture (shallow depth of field)
f.2.2

small aperture
deeper depth of field
f/8

Shutter Speed:

• The slower your shutter speed, the more light you get;
• The faster your shutter speed, the less light you get;
• In the viewfinder, 100 means a shutter speed of 1/100;
• Shutter speeds of 1/60 or less  cannot usually be handheld;
• For telephoto lenses, even faster shutter speeds are needed to handhold the camera, a rule of thumb is for a 100mm lens you need 1/100 or faster, 300mm = 1/300 or faster, 500mm = 1/500 or faster, etc.

ISO:

• SO determines how fast your image is recorded;
• ISO 100 will record light the slowest (less light);
• ISO 800 will record light 8x faster than ISO 100 (more light);
• Higher ISO's will result in more noise or "graininess" in your image.

• Each setting effects the others;
• You can test how aperture and shutter speed relate to one another in this camera simulator for awhile. And note that ISO relates to aperture and shutter speed in the same way.