Property is the set of things that a person can own. To say that a person owns something means that the person has the right to the exclusive control over how that object is used. A fantastic example of ownership is a human body. We all own our own bodies, that is to say we all have the right to the exclusive control over what our bodies do. To say otherwise is accept a contradiction in terms, since even slave masters can not actually control the slave. Although the slave master can indeed impose negative incentive structures on the slave to coerce the slave to doing his or her bidding, the slave must ultimately submit his or her body to the master's demands. Similarly each individual owns his or her life, labor, and less commonly appreciated his or her thoughts. No person can exercise the exclusive control over another's thoughts, and similarly each person has the exclusive control over what that person thinks.
Now with all this in mind we must ask: what is not property? Looking at the definition provided above we see that the opposite of property is the set of things that a person can Not possess the exclusive control over how that object is used. Thus people's bodies that are not our own are not property, since for me to own some one I must have the exclusive right over how that person uses the body in which they reside, and therefore must always either ask them, or impose sufficient negative incentives on them in order to get them to do what I want. I can't simply think and have them obey.
Another thing which is not property is the set of ideas, since no idea can be exclusively controlled. To understand why this is, consider a house that you have built. If I buy some land next to yours and build a house that is the same as yours in every detail, you still have your house, and I have my house, and we both have the exclusive right to control what is done with our respective houses. Now some would say that I've stolen from you in that I've stolen your idea. If that were the case me copying your house design would deprive you of your house design which is not the case at all.
Ideas have many interesting facets, for instance as we have just seen they can be copied exactly, but another facet of ideas is that two people can have the same idea at once. This is not true for property. You and I can not both own your body, but you and I can both realize that the ratio of a circle's circumference to it's diameter is the same for all circles. If this beautiful fact were property, did I steal it from you or did you steal it from me? Clearly the case must be that we both own this thought, but the idea can not be owned since neither of us can secure the exclusive control over the idea. Thus the idea can not be property.
Perhaps an example is in order. Suppose that you come along and say that you own an idea. I, having never thought of this idea, agree that you own this idea. If a third person overhears us, walks up to us, and claims that she has thought that idea as well, what are you to do? There is no way for either of you to prove ownership over this idea. What's worse she has your idea and so do you! Consider the case where you claim that you own a given tree. Again I, having never owned any tree, agree that this tree is yours, or at least not mine. If that same third person again walks up and claims that the tree is indeed hers, it may be possibly to establish who owns the tree. If you bought the tree and have a receipt of your purchase, this would be ample evidence of your ownership of that tree. Furthermore you would have the tree, and she would not, it's not as if she walked up with the same tree and claimed that both were hers.
Notice how silly it is to apply the same reasoning to ideas as we to do property: if some unscrupulous person were to come to you with ideas for sale, that person could sell you that idea, and turn right round and sell another person that same idea with out having to take the idea from you! How sad you would be to have payed for an exclusive right which can not exist! Instead we understand, quite rightly, that ideas can not be owned.
This example leads us to the only way ideas can be monetized, through the control over the transference of the idea. If the control over an transference of an idea is lost, then so to is any hope of financial gain from that idea. The same is not true for physical objects, it's true that if hooligans come and cut down my tree then I've lost control of it, but this is theft (the depravation of my property from me). This situation is different then one in which those same hooligans print out a copy of my tree and place it their own disorderly yard. However those hooligans have not deprived me of the use of the wood from my tree, for example. Again we see a way in which ideas are not the same as property.
While it is a mean trick for Blizzard Entertainment to sell a box with a CD inside this box labeled Diablo 3, and then require that the game be played only while being logged on to their servers: it is the only way that Blizzard can reasonably monetize the idea of that game. In the mean time I'll take pleasure in using the words "Diablo" and "three" in sequence while not being logged on to their servers.