In Theravada Buddhism ― the dominant school of southeast Asia ― it is thought there is only one buddha per age of humankind; each age is an unimaginably long time. The buddha of the current age is our historical Buddha, Siddhartha Gautama. Another person who realizes enlightenment within this age is not called buddha. Instead, he or she is an arhat (Sanskrit) or arahant (Pali) — “worthy one” or “perfected one.” The principal difference between an arhat and a buddha is that only a buddha is a world teacher, the one who opens the door for all others.
Arhat is a perfected person, one who has gained insight into the true nature of existence and has achieved nirvana (spiritual enlightenment). The arhat, having freed himself from the bonds of desire, will not be reborn in the current state of existence.
There are two main schools of Buddhism: Mahayana and Hinayana (Theravada).
Hinayana (Theravada) school focuses on finding perfect inner peace and want to live in a kind, truthful and generous way in the world. The end of their journey, their dream, their goal -the perfection of inner peace- is called the state of being an Arhat.
Sometimes you can achieve that state in one lifetime of intense meditation, sometimes it takes many lifetimes.
Mahayana Buddhists criticize the arhat ideal on the grounds that the bodhisattva is a higher goal of perfection, for the bodhisattva vows to become a buddha in order to work for the good of others. This divergence of opinion continues to be one of the fundamental differences between the Theravada and Mahayana traditions.
In the Theravada tradition, the spiritual ideal that is the realization of arahantship—the realization of nibbana (nirvana) and the ending of rebirth, in this very life.
Four stages of attainment are described in Pali texts:
The state of the “stream-enterer”—i.e., a convert—achieved by overcoming false beliefs and doubts regarding the Buddha, Dhamma, and Sangha.
The “once-returner”, who will be reborn only once in this realm, a state attained by diminishing lust, hatred, and illusion.
The “nonreturner”, who, after death, will be reborn in a higher heaven, where he will become an arhat, a state attained by overcoming sensuous desire and ill will, and...
Except under extraordinary circumstances, a man or woman can become an arhat only while a monk or nun.
In China, Korea, Japan, and Tibet, arhats were often depicted on the walls of temples in groups of 16. They represent 16 close disciples of the Buddha who were entrusted by him to remain in the world and not to enter nirvana until the coming of the next buddha, in order to provide people with objects of worship.