Who is Buddha?
As mentioned above Buddha can refer to the historical Buddha Shakyamuni or to anyone who has attained full enlightenment. Geshe Kelsang Gyatso explains in Introduction to Buddhism:
In general, 'Buddha' means 'Awakened One', someone who has awakened from the sleep of ignorance and sees things as they really are. A Buddha is a person who is completely free from all faults and mental obstructions. There are many people who have become Buddhas in the past, and many people will become Buddhas in the future….There is nothing that Buddha does not know. Because he has awakened from the sleep of ignorance and has removed all obstructions from his mind, he knows everything of the past, present, and future, directly and simultaneously. Moreover, Buddha has great compassion which is completely impartial, embracing all living beings without discrimination. He benefits all living beings without exception by emanating various forms throughout the universe, and by bestowing his blessings on their minds. Through receiving Buddha's blessings, all being, even the lowliest animals, sometimes develop peaceful and virtuous states of mind. Eventually, through meeting an emanation of Buddha in the form of a Spiritual Guide, everyone will have the opportunity to enter the path to liberation and enlightenment. As the great Indian Buddhist scholar Nagarjuna said, there is no one who has not received help from Buddha.
It is impossible to describe all the good qualities of a Buddha. A Buddha's compassion, wisdom, and power are completely beyond conception. With nothing left to obscure his mind, he sees all phenomena throughout the universe as clearly as he sees a jewel held in the palm of his hand. Through the force of his or her compassion, a Buddha spontaneously does whatever is appropriate to benefit others. He has no need to think about what is the best way to help living beings - he naturally and effortlessly acts in the most beneficial way. Just as the sun does not need to motivate itself to radiate light and heat but does so simply because light and heat are its very nature, so a Buddha does not need to motivate himself to benefit others but does so simply because being beneficial is his very nature.
Like the reflections of the moon that effortlessly appear in any body of still water, a Buddha's emanations spontaneously appear wherever living beings' minds are capable of perceiving them. Buddhas can emanate in any form whatsoever to help living beings. Sometimes they manifest as Buddhists and sometimes as non-Buddhists. They can manifest as women or men, monarchs or tramps, law-abiding citizens or criminals. They can even manifest as animals, as wind or rain, or as mountains or islands. Unless we are a Buddha ourself we cannot possibly say who or what is an emanation of a Buddha.
Of all the ways in which a Buddha helps living beings, the supreme way is by emanation as a Spiritual Guide. Through his or her teachings and immaculate example, an authentic Spiritual Guide leas his or her disciples along the spiritual path to liberation and enlightenment. If we meet a qualified Mahayana Spiritual Guide and put into practice everything he or she teaches, we shall definitely attain full enlightenment and become a Conqueror Buddha. We shall then be in a position to repay the kindness of all living beings by liberating them from the sufferings of samsara and leading them to the supreme bliss of Buddhahood.