We begin our practice by taking refuge in the Buddha, the Dharma, and the Sangha.
Buddha, the Enlightened One:
Originally, Gautama Siddhartha (Shakyamuni), the founder of Buddhism, was called by the name Buddha, as he was the one who had attained the state of Enlightenment at 35 about 2,500 years ago in India.  The final goal for all Buddhists, irrespective of sect or culture, is to become a Buddha. 
Dharma, the Teaching:
Dharma (or Damma) is the teaching given by the Enlightened One, the Buddha.  There are three types of canons: Sutras (teachings by the Buddha Himself), Vinayas (disciplines provided by Buddha), and Abhidharmas (commentaries and discussions on the Sutras and Vinayas by scholars in later periods.)
Sangha, the Buddhist Fellowship:
Sangha refers to the community of monks, nuns, and lay persons. In early times, it consisted of homeless monks and nuns.  Later, when the Mahayana movement arose, those who aimed at the state of Bodhisattva, regardless of being layman or monk, joined together in fellowship.