In addition to targeting all the major muscles of your abs, planks also hit the stabilizing muscles of your spine, shoulders, and hips while statically challenging your chest, triceps, and quads. In other words, they help tighten up just about everything.
That said, if you have a weak low back and core, or you're carrying a few extra pounds (and haven't hit the gym in a while), you'll want to start small. Try holding a high plank (like a push-up position) for 10 to 15 seconds, rest, then repeat three to five more times. Do this every single day, and each day, try to add five to 10 seconds to your goal. Once you can hold three sets of 30 seconds in a high-plank position, go ahead and progress to a forearm plank, a variation that taxes your core a little more.