Ownership is properly understood not as something that exists between people and things, but rather between people. It's an agreement not to touch stuff which we attribute as "belonging" to other people, and for them to similarly avoid messing with things that are attributed to us. If this takes the form of a truly voluntary agreement, then it is constantly renegotiable, the distribution of ownership being determined by a consensus of concerned parties, which in most cases means through the consensus of a given community. Communal peasant societies, for example, would redistribute land on a yearly basis to the extent that it was divided, and utilise what remained collectively. What's mine is mine because we have agreed that this is so.
Private property, however, is not renegotiable; that is what makes it historically significant. It is not a claim of access or utilisation derived from the community, but a claim of absolute disposal; it is in principle asserted by the individual, rather than agreed upon by the community. Because this is not a consensual form of ownership-attribution, not something that we actively volunteer to participate in, it is therefore something that must be enforced; an agreement that we are threatened with or subjected to violence if we fail to adhere to. If I pirate a CD, for example, I may have violence levied against me in the form of arrest and even imprisonment, regardless of whether or not I ever gave the barest suggestion that I accepted the authority of the publisher and the state to impose its intellectual property legislation upon me.
Thus, property is a relationship of domination, a command to act in a certain way- to respect certain property claims- backed up by violence. As a libertarian, I oppose all forms of domination, regarding only voluntary relationships as acceptable for human beings, and so cannot but reject property is a petty tyranny.