Property damage is damage incurred upon property which somehow reduces the overall quality of that property. Property damage might not, of necessity, lead to a decrease in the functionality of that property, but it would still lead to some decrease in the overall value of the property with all likelihood. For example, a house might suffer property damage in the form of scratches along the walls of the house, either inside or out.
This property damage might not prevent the house from offering up the benefits of a house, being specifically shelter and security, but the house would be less attractive and therefore less valuable thanks to the property damage. Property damage to a house might also include more serious damage, however, such as property damage to a door or window. Property damage might be caused by natural events, or by an outside actor.
Property damage is generally not the term used when that damage is incurred purposefully by the property's owner, as technically the property's owner does have the right to change and affect his or her own property. Instead, property damage is the term applied for those instances when a property is damaged such that the owner might be able to obtain some form of restitution for the property damage incurred, depending on the particular situation.
For example, if an individual caused the property damage, then the owner might be able to obtain compensation from that individual. If the property damage were caused by a storm, then the owner might be able to obtain compensation from some form of storm insurance.