AIRNow.gov deals with concentrations of ozone in the lower part of the atmosphere, where ozone is harmful for people to breathe.
An ozone "hole" refers to a place where ozone in the upper part of the atmosphere has been depleted by the use of certain chemicals. The ozone in the upper atmosphere (the stratosphere) is what protects the Earth's surface from ultraviolet radiation. The increased ultraviolet radiation at the Earth's surface may increase skin cancer and eye cataracts.
For more information about ozone depletion, please see http://www.epa.gov/ozone/strathome.html.
For more information about ozone in the lower part of the atmosphere versus ozone in the upper part of the atmosphere, please see Good Up High - Bad Nearby.