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Levels is a smarter though a bit more complex tool than Brightness/Contrast. Try it now.
1. Open your image.
2. Pull down menu Image > Adjustments > Levels... A dialog box appears.
3. Test the Input Levels (the upper sliders) first. To adjust image brightness,
move the medium (midtones) 
slider.
4. To add some contrast, darken the shadows with the "dark" 
slider
and/or brighten the highlights using the "light" 
slider. In most cases, to adjust contrast roughly, we have to move the shadows and
highlights sliders to "cut
off" the lowest parts of the histogram graph (in this example - the "highlights"). Sometimes it's called, "stretching" histogram.
NB If something is going wrong, undo the last step [Ctrl+Z] or Alt-click Cancel (hold down [Alt] key and then click Reset button) to start adjustments from the beginning.
5. Now test the Output Levels (the lower) sliders. It seems, they decrease contrast, don't they? Use the Output Levels to optimize your images for an output device like printer or commercial press. Shifting one or both sliders by 5 - 15 levels, may prevent loosing some important detail in the darkest and brightest areas on the printed pictures.
Tip. If you feel confused about the Levels, just keep your mind off this for a while and use Brightness/Contrast. Some day you will come back to the Levels as a skilled user.
