The intuitive notion of property is some concrete, physical thing that can be hoarded or shared. Intuitively, the boundaries of property can be marked and patrolled, and those who would violate it by theft or invasion can be easily seen and straightforwardly stopped. In the realm of intellectual property, however, borders are anything but visible, and attackers are anything but straightforward. Instead of being made of nuts and bolts, they are made of the blood, sweat, and tears of their creators. You may not be able to hold it in your hand, but intellectual property has unquestionable value nonetheless - and not just in the almost maternal way that an artist might value his painting or a writer her novel.
Intellectual property has real market worth as well.
The Marlboro trademark, for instance, is the most valuable piece of intellectual property in the world. In legal terms, property is not the object itself. It is not the book you hold in your hand, not the car that you drive to work. Property is the rules and relationships that society has built up around those objects, the set of ways that you and everyone else relate to owned things. Property is a set of rights that govern the way other people treat your things and the way you treat other people's things.