In Theravada (Southern) Buddhist countries, the monks (bhikkhus) are easily recognized because they wear the characteristic orange robe, have their heads shaven, and go about barefoot. They are given a new name and the robe, and will live according to a code of 227 rules (the Vinaya). A monk may decide to disrobe (cease being a monk) at any time.
Bhikkhus live a strict, simple life of meditation, study and work, with very short hours of sleep and only one main meal a day. They do not own money or any possessions. They help with the important task of teaching and assisting lay people, and conducting ceremonies.
In Mahayana (Northern) Buddhist countries there are two main branches, the Tibetan with monks wearing the characteristic maroon robe, and the Far Eastern Zen, which also has an unbroken line of nuns, where the robes are black or grey.