In England and Wales, a common is a piece of land over which other people -- often neighbouring landowners -- could exercise one of a number of traditional rights, such as allowing their cattle to graze upon it. The older texts use the word common to denote any such right, but more modern usage is to refer to particular rights of common, and to reserve the word "common" for the land over which the rights are exercised.
The fact that land is common land does not mean it has no owner -- all land in England and Wales is owned by someone. Those who have a right to exercise a right of common are known as 'commoners. Historically most rights of common were "appurtenant" to particular plots of land. So the commoner would be the person who, for the time being, was the owner of the land. Some rights of common were said to be "in gross" in that they were unconnected with ownership or tenure of land. This was more common in regions where commons were more extensive, such as in Northern England or in the Fens.
Example rights of common are:
common pasture (right to pasture cattle, horses, sheep or other animals on the
common land)common piscary (the right to fish
common turbary (the right to take sods of turf)
estovers (the right to take sufficient wood for the commoner's house or agriculture)
Rights of common grazing might also be held over privately owned arable strips in some open field manors; this right was excercised either after the harvest or during a fallow year. Rights of grazing in the open fields was most valuable in manors with relatively little other grazing.
It is often thought that a common is somehow owned by everyone, or at least by "the community" in some sense. While that may have been true more than a thousand years ago, when waste would be used for grazing by the local community and over which there would not be, nor would there need to be, any particular limit or control of usage; since at least late Anglo-Saxon times, the right to exercise a right of common has been restricted to a commoner. Since the right of common would have some natural limitations itself, commons never suffered from the tragedy of the commons.
Thus the word is now used in the sense of any sets of resources that a community recognizes as being accessible to any member of that community. The nature of commons is different in different communities, but they often include cultural resources and natural resources.
While commons are generally seen as a system opposed to private property, they have been combined in the idea of "common property", which are resources "owned" equally by every member of the community, even though the community recognises that only a limited number of members may use the resource at any given time.
The act of transferring resources from the commons to individual ownership is known as "inclosure."
Commons are a subset of public goods; specifically meaning a public good which is not infinite. Commons can therefore be land, rivers and, arguably, money. "The Commons" is most often a finite but replenishable resource, which requires responsible use in order to remain available. A subset of this is a commons which requires not only responsible use but also active contribution from its users, such as a school or church funded by local donations.
In order to ensure responsibility of the users, there must be a system of management. Such models include the Hobbesian "Leviathan" model, where there is a central authority that monitors the behaviour of the users and can sanction abusers. There are also many other models, some of which can require no maintenance -- for instance, if it is known that the collective consists mostly of contingent cooperators, then once responsible behaviour has been established, it will most likely continue without management. Another model is reputation management.
"Commons", Wikipedia, http://en.wikipedia.org/wiki/Commons