The community of the followers of the Buddha's teaching. Different interpretations are given for the sangha. In some versions, it consists only of followers of the Buddha who have attainted a certain measure of insight (usually the stage known as sravika or translated 'stream winner' or 'voice hearer', depending on the scholastic mood). In other instances, it is anyone who has taken refuge in the Triple Gem (tiratana). In the traditional Theravada conception, the Sangha consists of four catagories of people: ordained male followers (monk/bhikkhu), ordained female followers (nun/bhikkuni), lay male followers, and lay female followers.
Along with the Buddha and the Dhamma, the Sangha is one of the three traditional sources of refuge for Buddhist followers, also called the Triple Gem, Three Jewels, or Three Treasures (Pali: tiratana). The responsabilities of the Sangha including preserving and teaching the messege of the Buddha, through study, recitation, and preservation of the suttas, ordaining new monks and nuns in order to continue the linneage, and serving as a field of merit for lay followers.
This last component is not to be ignored; for lay Buddhists, particularly in Theravada societies, the most important thing that the Sangha does (at least in the view of the laity) is to provide them with an opportunity to perform good kamma (Sanskrit: karma) by giving gifts to the Sangha, and supporting them materially. In return, the Sangha has an obligation to maintain an outstanding moral character, to improve the efficacy of the generosity of the laity, and to ensure that lay followers and new monks and nuns are provided with good examples of correct moral conduct. Sila, or morality, is seen as the underlying foundation of all other Buddhist practices, such as meditation and stufy, It is the responsability of the Sangha to cultivate this foundation among both lay and ordained followers. Many of the regulations contained in the Vinaya are oriented towards ensuring that lay followers maintain their faith in the morality and propriety of the Sangha.
This creates an interesting balance between ordained and lay Buddhists. On the one hand, ordained Buddhists have almost total control of access to the higher teachings of Buddhism, those found in the Sutta Pitaka and the Abhidhamma. Teachings regarding meditation have traditionally been regarded as the regime of the ordained practitioner as well, though this is slowly changing. So laymen are dependent upon the monks to supply them with the teachings that will ensure their success in the quest for a better rebirth, and ultimately for Nibbana. But, because most lay followers percieve that their generosity to the Sangha is less useful if the Sangha is not morally upright, and because monks and nuns usually depend on the dana of the laity for their material preservation (food, clothing, and shelter are all provided by the laity), monks must always strive to prove their propriety to their lay followers.
During times of unrest, when the ordained Sangha is perceived as having become corrupt and amoral, lay followers have been known to withold food from certain monks or groups of monks. This effectively forces the monks to either change their behavior, leave ordained life, or starve! Such practices occured in Sri Lanka during the Buddhist revival and the Forest monk movement, and has occured from time to time in other areas as well. In this way, feedback is created between the laity and the ordained Sangha. This relationship was intentionally maintained by the Buddha, who rejected a number of proposals that would have permitted monks to become self-sustaining for exactly this reason.
In modern times, deviations from the old practices have meant that this power dynamic is increasingly breaking down, usually in the favor of the established Sangha of a given state or region. Monasteries and temples have become land owners of significant size, and are much less dependent on the laity for their upkeep. This was particularly true during medieval times in East Asia. On the other hand, due to the expansion of literacy, the translation of Buddhist scriptures into local languages, and new communications technologies, the more complex Buddhist teachings are no longer reserved for monks and nuns alone. So while the ordained Sangha is now less dependent on the laity for their material upkeep, the lay Sangha is also now less dependent on the ordained members for their spiritual and moral progress
How the situation will play out remains to be seen. Allegations of corruption are leveled at the ordained Sangha in every generation. Modifications to the Vinaya rules in many areas, such as the addition of marriage and the ability to own property in many East Asian countries (Japan at one time required all monks to marry) means that the line between the lay and ordained Sangha is becoming more and more blurred. Even in the more traditional Theravada world, lay teachers of meditation, Pali, and other traditionally monastic topics are increasingly being accepted. And who knows what goes on in the West, where the absence of traditional structures and a cultural rift between 'convert' and 'ethnic' Buddhists has further confused the situation. The Sangha has a good shot at laying claim to the title of the world's oldest organization (they predate the Catholic Church, which usually gets the honor, by a goodly number of centuries). How it will change in response to the challenges of the modern world remains to be seen. | 
