The Buddha is the ultimate ideal of enlightenment. Although the historical Sakyamuni and His emanations are the personification of enlightenment, and He is praised with the honor and esteem of the the Ten Titles, the Buddha really is totally beyond any aspects of personality or individuality. The Buddha has attained the highest ideal of Nirvana, and so all traces of egocentrism have been extinguished. The Buddha has truly conquered Life & Death.
As the Bodhisattva practice is the cause of enlightenment, the different stages of Bodhisattva development give a clue as to the real nature of the Buddha:
With the The Abodes one opens up and inspires an understanding of the nature of enlightenment
With the Ten Practices of Virtue (the Paramitas), one reveals the nature of enlightenment in one's own life.
With the Ten Transfers of Spiritual Merit (Parinamanas), one awakens the nature of enlightenment in others.
With the Ten Groundings, there is entry into nature of enlightenment that beyond self or other.
The Buddha is beyond all limitations of self or other, and is the unconditional will and means to relieve suffering and bring enlightenment to all living beings.
The nature of the Buddha is revealed in two aspects and three aspects.
The Two Aspects:
The Manifestation of the Buddha in this world; The life of enlightenment in the flesh, in the realm of Life & Death; This is the incarnation of enlightenment (Rupakaya or Nirmanakaya), the appearance of the Buddha in this world in response to the needs of living beings for enlightenment and the relief of suffering. This is personified by the life of Sakyamuni and its story as an allegory for the life of enlightenment.
The Origin or Source of the Buddha in the spiritual realm; This is the spiritual body (Dharmakaya), the eternal nature of enlightenment that is beyond space or time and is innate in all living beings. This is the eternal Buddha that is the spiritual truth of reality that is everywhere in all places, illuminating and teaching all.
The Three Aspects of the Buddha's Life:
The Physical Manifestation of the Buddha in this world (Nirmanakaya);
This is often represented by His 32 Ideal Physical Characteristics. This is the aspect of the Buddha that is easiest to relate to, as it is the anthropomorphic representation of the Buddha. This is the Buddha that appears in this world before our eyes in response to our need for salvation and enlightenment. This would also refer to any physical representation of the Buddha that we can detect with our senses (statues, images, the sounds of the Dharma being taught, etc.).
The Wisdom and Virtues of the Buddha (Sambhogakaya); This is the enlightenment that is earned through vows, practices and sacrifices made over countless lifetimes.
It is often expressed as the Ten Spiritual Powers,
the Four Kinds of Fearlessness,
the Four Unhindred Forms of Expression,
the Eighteen Unique Qualities, etc. -
see the Forty Unique Qualities of the Buddha for an overview of these. Through worship and service to the Buddha these rewards of spiritual merit are transferred from the Buddhas to the Bodhisattvas. This is the enlightenment that is conferred through the ceremony of the Three Treasures.
The Original Buddha that is the Spiritual Body (Dharmakaya); This is the unity and trinity that is the Innate Threefold Body of Enlightenment that is eternal and everywhere in all places.