On there most basic level, a commons refers to one way in which an institution determines the rights to a)access, b)use, and c)control its resources (Benkler 60). The obvious alternative form is the property based system. As Benkler puts it, the fundamental difference between the two is as follows: "No single person has exclusive control over the use and disposition of any particular resource in the commons" (Benkler 60). But this does not necessarily mean that anyone who wants to can use/access/control said resource nor does it mean that he can do so as he sees fit without having to respect certain rules. Commons very in terms of how heavily they regulate who can have access to a particular resource and how one can use/control it.
On one end of the spectrum would be a totally "open-access" commons in which there is no higher authority to control who uses it (which in a property based system often means charging money for it) or how they use it. A completely "open" commons is hard to find. One explanation for this scarcity that Benkler touches on is this notion of "social-conventional" restrictions (Benkler 62). In less criptic terms, people often regulate themselves simply because that's how they were raise or because they don't want to be looked down upon by others. Take our dining halls, for example. There is no militant dishwasher holding a gun to our heads screaming "don't take plates to your room and not return them," but I usually bring them back because I know I personally hate it when I have to eat pasta with a spoon and I'm others do too. I often get dirty looks from my friends. Most people aren't economists, but they still understand the concept of "rival" goods. The e-mails threatening to cancel midnite breakfasts if dishes aren't returned are constant reminders that "open-access" is a privilege and can be revoked if abused.
On the other hand of the spectrum is what Benkler calls "limited-access" commons (61). This MediaWiki that we are editing is a perfect example of such a system. For one thing, we can't even edit this without first entering our Middlebury user name and password. Access is further limited to members of our class (I think - correct me if i'm wrong). Secondly, certain pages are locked from editing...if I wanted to add another term to the list that I feel needs to be there, I am physically prevented from doing so by the "formal" restraint of the software itself. Despite the varying level constraint amongst commons-based system, the rules are the same for each and everyone of us. As Benkler points out, these restrictions are "symmetric" for every user (62). One final distinction that must be made is that not while peer production is inherently a common based system, not all commons are examples of peer production. Refer to that article for clarification.
Yochai Benkler, The wealth of networks : how social production transforms markets and freedom (New Haven: Yale University Press, 2006)