Reading the Histogram
Many times you take a photo and when you look at it on your computer it looks darker or brighter than you intended. This is because the LCD on the back of the camera cannot always display colors as they would appear when enlarged, and you might be looking at the LCD in bright sunlight.
This is why digital cameras can display a graphic that shows the levels of light in your scene. This is called the histogram. To find it on your camera, you might need to press "display" while in preview mode. (depending on make or model)
(image: overexposed photo with detail lost in the bright parts)
The first thing to know when reading the histogram is that the left edge represents the shadows, the right is the highlights and the middle is the midtones. The shape of the histogram must go up from the left and right edges and not be cut off.
The area of the curve that is cut off, represents detail that is lost in the dark or light areas of the scene. This is especially important when taking photos of wedding dresses, where you want all the details of the dress to show and not just a white sheet of fabric.
Another example is when shooting outdoors with some parts in shadows and others in bright sunlight. An easy fix can be to move the subject into shade or alter the composition of your scene. This would save you a lot of time in editing later.
All this could be missed if you didn't notice your histogram reading.
Fixing in Photoshop
If however your scene is such that you couldn't move the subject, or you missed something on the shoot, you can still bring back some detail in those areas, but only to an extent. This is done by using the "Levels" Adjustment in Photoshop and simply adding a mask. You then make the mask black with the fill tool, and paint in white over the areas where you want the adjustment to happen.
A tip to use for all your images, is to check the histogram in the "Levels" adjustment and then dragging the right slider back to the point where the histogram starts to appear from the bottom. This fixes images that are slightly under exposed and will not result in any loss of detail.
If you are editing images in Black & White, the "Levels" Adjustment is very handy in adding some contrast by dragging the left slider slightly to the middle.
Why not just edit?
Photoshop has made our lives as photographers a whole lot easier, yet we have become reliant on it to do most the work for us. In the end, a bad image can only go so far. So try getting it right in camera first, and see your photos improve to the next level.