Ozone is "good" when it is in the stratosphere. The stratosphere is a layer of the atmosphere starting at the level of about 6 miles (about 10 kilometers) above sea level. The stratosphere naturally contains about six parts per million of ozone, and this ozone is very beneficial because it absorbs UV radiation and prevents it from reaching us (see How Sunburns and Sun Tans Work for a description of the damage that UV radiation can cause to skin). Check out this page for a good description of the chemical reaction that allows ozone to absorb UV light.