Originally referred to the Indian prince Siddhartha Gautama who was supposed to have lived in India near the modern border with Nepal and to have become enlighted at the age of 35, but later applied to other people who are supposed to have become enlightened. Gautama's enlightenment was the realisation that the main source of dissatisfaction ('dukkha') was clinging to self, things and desires, which are all empty of independent existence ('sunyata'). Confusion about the nature of things, for example the belief in a separate self is described as 'samsara' and freedom from it as 'nirvana'.
Contrary to common Western belief, Gautama Buddha did not invent the idea of reincarnation (this was the dominant belief at the time) and his philosophy taught liberation from both reincarnation and dependence on worshipping gods. Belief in reincarnation or other metaphysical ideas is unnecessary.
Boy, people have a lot of wrong ideas about Buddha, they even think he was fat, when in fact that was a Chinese monk who later became known as the Laughing Buddha.