A ball rolls because some force, such as gravity, the wind or a person's foot, acts upon it, setting it into motion. It continues to roll until some other force, such as friction, gravity or a barricade of some sort, causes it to stop.
Rolling is a type of motion that involves rotation and translation, which is what physicists call the motion of a body from one place to another. Rolling occurs when the rotational speed matches the translational speed, meaning no sliding. Rolling requires less energy than sliding because rolling resistance is less than sliding friction. However, on a perfectly frictionless surface, a ball would not roll at all. It would slide instead.