In both Theravada and Mahayana Buddhism, buddhas are those who have attained full enlightenment. Siddharta Guatama became "the Buddha" after his Enlightenment under the Bodhi Tree.
Theravada Buddhism recognizes only Gautama and the past Buddhas, but in Mahayana Buddhism, everyone has the potential to become a buddha; i.e., an "enlightened one." (Compare Theravada and Mahayana Buddhism)
Mahayana Buddhism thus includes belief in numerous "celestial Buddhas" who have attained enlightenment and now dwell in the heavens or the various Paradises.
Because of their enlightened state, Buddhas are generally believed to be unapproachable, but there are still perceived benefits to venerating or meditating on them and their accomplishments.
Celestial Buddhas,especially the Five Dhyani Buddhas, play a major role in the Tibetan and Tantric forms of Buddhism.