This is possibly one of the most widely used and most simple method to adjust an image. It's so simple and effective that you will want to use it on all your pictures from now on, so keep an eye on this, and have fun.
Lets see this sample photo from Akademy 2008, kindly donated by Sebastian Kügler:
You'll clearly see, the photo is darkish. After all, it was taken during a presentation and possibly the room was pretty dark. But that's no excuse for a bad photo =)