Siddhartha Gautama is the religious founder or leader of the religion Buddhism. In that sense he is the leader. As far as a living leader of all Buddhists, there is none. Like Christianity each sect has their own hierarchy and leaders.
2,500 years ago by a prince named Siddhartha Guatama (Buddha).
This belief orginates from the Hindu religion and is not an accepted part of Buddhist thought.
The reason that this is not part of Buddhist thought is that the Buddha lived as an ascetic and fasted for weeks at a time. He reached the point where his body was very emaciated and he was on the verge of death. At this point he was offered some food by a passing person (watery rice, I believe). The ascetics he lived with in the forest belived he had betrayed their cause and refused to have anything to do with him (some apparently later changed their minds). From this he was able to realise that complete deprivation is NOT the way to live.
Scientific research undertaken in Australia has since proved that this is NOT possible. See the link on Breatharianism for further information.
Buddhism is practised all over the world. It is especially common in South-East Asia, in Tibet, Bhutan, Thailand, Burma, Nepal, Laos, Cambodia, Vietnam, and others. It is also becoming more common in the Western world, however.