In canonical terms, the Pali term sangha refers to the community of Theravada Buddhist bhikkhu (monks) and bhikkhuni (nuns); it can also be taken to denote, in the ideal realm, followers of the Buddha who are on the path to enlightenment - stream enterers, in English. More recently sangha has taken on a popular meaning of the entire community, lay and ordained, of Buddhist followers, though parisa is actually the correct term for this larger group.
In countries like Thailand, where the sangha is a large and organized group of bhikkhu (though not bhikkhuni), the sangha is controlled by a powerful overseeing group which has the power to defrock bhikkhu who they believe are behaving improperly.