If you shoot in RAW format, you have some options to control contrast in Camera RAW Import in Photoshop (You can also
use Lightroom, GIMP, or other tools). The sliders in the import interface allow you to control the entire image. The other option you have is the Adjustment Brush built into Camera Raw, which allows you to control contrast by painting specific areas darker or lighter. Adobe Lightroom provides similar tools.
There are many kinds of contrast, but we are mostly concerned with tonal contrast, the difference between predominantly light and dark areas in a photograph. An image with large amounts of black or near-black areas, as well as large amounts of white or very light area, is said to be high contrast. An image that has mostly middle grays is said to be low contrast, even if it has some minor areas of extreme lights and darks. One of the easiest ways to make your images look pleasing is to increase the contrast. If someone says an image “pops,” contrast is often one of the elements they are talking about. An image with good contrast has plenty of bright highlights and rich dark shadows.
High contrast is only one option, there may be times when you want the effect of contrast that is more subdued.
You affect contrast with your initial exposure. Highlights that are too light to show enough detail are called blown out;
shadows that are too dark to show enough detail are called blocked up.
The above information is from True Confessions of Nude Photography.