Buddhism can be broken down into mainly Theravada Buddhism and Mahayana Buddhism. Each sect has its own type of Buddha as well as concepts according to the doctrine. In Theravada Buddhism, anyone who realizes and accepts the true nature of reality is called a Buddha. A Buddha comes to a point where he no longer goes through the cycle of birth and rebirth. In Mahayana Buddhism, the Buddha is typically not human but rather an earthly representation of the endless being.
It is considered that there has been a Buddha for different eras throughout the course of human existence. The Buddha of the current era is Siddhartha Gautama. Buddhists believe that Siddhartha Gautama was the first to achieve enlightenment in the current era and ultimately became the Buddha of this era.