"Curves" as we'll call it from now on, is a very common tool, present in most photo manipulation programs. If you don't have Photoshop or Lightroom, it's ok - I actually prefer to use GIMP, a free Photoshop-like software that has TONS of capabilities.
This Instructable is about using curves for a specific purpose - getting the details out of the shadows in a picture, but you can check out the full tutorial at my article, Curves Tool - Full Control over Tones and Contrast
Step 1: First, a Bit About Images and Sensors
Whether you're shooting with a DSLR, point & shoot, or a cell phone camera, it all happens the same. The light is recorded for a certain period of time, then the image is pushed to the memory of the camera.
If you have the capability to shoot in RAW, you have the best possible chance of getting good results because RAW files are huge files that record every bit of light without compression. But if you don't have the capability, it's ok - jpegs still capture 16.7 million colors, which is plenty to extract and image from.
Step 2: Choose a Picture With Lots of Dark Shadows
When I took this picture, it looked like 3 completely darkened silhouettes against the sky - cameras just do that sometimes, especially when you're not in full manual mode for complete control over the outcome.
Again with the colors, JPEG images can record 16.7 million colors. This picture may look like it's just orange and black, but it's really light orange, lighter orange, even lighter orange, a bit lighter orange, and so forth. The black section is really probably about 10-100 different shades of really dark gray, red, blue, green - you name it - which is plenty. Your eyes just can't tell the difference as-is because the colors are all so close together.
Step 3: Open the Image in Your Editor
The curves too is actually a graph of the lightness and darkness of your image. The horizontal axis (x-axis) represents all the possible colors of gray from pure black to pure white that could be in your image. If you look closely, you can even see a lightly colored histogram used as the background of the cart. I've tagged the image with the meaning of the histogram.
The vertical axis (y-axis) represents how those parts of the image will show up when you're done. In other words, it's an input vs output graph. The black line going through the middle means the blacks look black, the grays look gray, and the whites look white.
Step 4: Start Pulling on the Curve
Add one control point right in the center of the curve. This will anchor that part of the curve there so you can manipulate the left half (the shadows) without changing the right half (the highlights).
Add another control point between the left and right halves and pull it upwards - this will cause the darker parts of the image to become lighter.
This one took a few tries, and multiple applications of the same technique, but eventually I got it. For a full tutorial on using the tone curves tool, visit my photography tutorial website, Picture Like This, or just take this link directly to the article:
Curves Tool - Full Control over Contrast and Tones