A crack, is simply a stone damage that has spread. Typically, the damage is near the edge of the windshield, and due to the stresses that a windshield goes through, such as heat or cold, or flexing of the car as it goes over bumps or around turns, will cause those rock chips to propagate.
A crack is different than a normal stone chip. A crack extends from the surface of the glass down to the plastic inner layer that is sandwiched between the inside and outside layer of glass, where as stone damages have one entry point, called the impact crater, and the damage is under the surface of the glass, like a tunnel. Windshields are laminated glass. It is usually the outer layer that incurs the damage.
Because a crack is open at the surface along its entire length, dust and dirt works its way into the damage over time much more readily than it does with stars, bullseyes, clam shell shaped, or compound stone damages. The older it is, the dirtier it becomes.
I am one of the only shops that know how to repair cracks up to 18″.