What you are seeing on the LCD screen is not necessarily what is going to show up on your
computer. It is really easy to do a whole series of photos and get home only to discover that your
masterpiece is under or over exposed and not at all what you were expecting.
Sometimes it can be fixed (after hours and hours in Photoshop) sometimes it can't. If the shadow
or highlight details weren't captured - they just aren't there to be "fixed".
Rule of thumb... Use your LCD display to check for composition. Use your histogram to check
for exposure.
The initial step to understanding how to use a histogram is to drag out your camera's manual and
figure out how to turn it on in playback mode so that you can see both the histogram and the
picture.
Reading your manual could easily be the most complex part of the operation. This is another of
those times when reading the manual (ten minutes of sheer boredom) can dramatically improve
your photography - forever.
Once you've learned how to turn on your histogram, take a few shots of something. For this test
run, find a subject that is mostly neutral in tone and not mostly dark or light. 18% gray cards are
great subjects and are sold for this very purpose!