The Photoshop Curves Tool is one of the most useful and one that I use with nearly all my photographs.
The levels tool helps to adjust the contrast in an image by seamlessly adding black and white tones.
The curves tool builds on that by also boosting the mid-tone contrast ranges, whilst also being able to provide more subtle adjustments to the shadows and highlights and increasing the brightness if required too.
Take a look at the picture below it is rather dull and grey and the definition of the reflection isn't very strong.
You can see from the levels palette that there is tonal range missing from both the left and right hand side. Meaning that there are no bright white tones in the image or deep black tones.
By sliding the bars on the right and the left inward until they touch the first area where tone has registered, you can see from the picture that there is now far more contrast in the image.
Now it is time to use the curves tool to take it that one step further.
Hit Ctrl+M and the curves palette should appear.
Now click and hold on the diagonal line and drag the line into the curve as shown. This will give the image even more depth of tone. You can adjust the amount of highlight or shadow to suit to your particular image.
You will notice that by dragging the line upward from this point you are adding more highlight and dragging down adds more shadow. A gentle S shape is often all that is needed.
You can now see how the Photoshop curves tool has affected our original image and has turned it from a dull lifeless flat picture into a much stronger more defined one.
The curves tool is very powerful and by playing around with the curves tool you will see how it can dramatically change the tones. Even to the extent of totally reversing the image and creating a negative version, shown here.