The founder of present age Buddhism is Shakyamuni Buddha. He was born in a royal family in ancient India. His biography had been recorded by his followers and passed down through the ages. In these records, we encounter many miraculous activities or supernatural events.
Some Buddhist prefers to ignore all supernatural narrations and looked upon the Buddha as an ordinary human being with great wisdom. In this respect, He appeals to those who wish to lead their life in accordance to the wise word of an enlightened teacher.
Some Buddhists believe that Shakyamuni Buddha is an enlightened super human being. In scriptural records, the Buddha possessed a human body unlike ordinary man. This is due to his good karma accumulated in countless previous lifes. He is capable of walking immediately after birth and possesses great strength and other unique characteristics in his physiques.
Shakyamuni Buddha is known as the historical Buddha. His remains and relics are scattered across the world and worshipped by Buddhists. In another word, Shakyamuni Buddha is not a fictitious character. The sub-menu Sakyamuni Buddha contains articles relating to Him.
Shakyamuni Buddha taught that he is not the only Buddha. There were Buddha(s) before him and there will be Buddha after him. We can all become Buddha if we try hard enough. In his sermons, he narrated and recounted different Buddha before him and left accounts of the next Buddha that will appear.
Such narrations relates to Buddha(s) that had appeared many eons ago or Buddha(s) that appeared in other world systems (universe, planes of existence etc) Just like how we describe our friends to someone who have never met them. Shakyamuni Buddha recounted how these Buddha started their quest for enlightenment, what was their main drive (example, to rid beings from ailments and disease, provide beings a pure land to reside while training for enlightenment etc)
These Buddha(s) are not from our present age or world. Unlike Sakyamuni buddha, we cannot relate to them physically (No relics, historical landmarks etc) Therefore, Buddhist scholars classified them as Celestial Buddha.
In the profound world of Buddhist scriptures, Sakyamuni Buddha would be a celestial Buddha to beings in another world or time too. Just like how He introduced us to the presence of these Buddha(s). They had also introduced Shakyamuni Buddha to their audience.
When reading Buddhist scriptures, there are narrations of how Buddha(s) from different times or different world systems can meet up. In the state of enlightenment, it seems like time and distance no longer constrain an enlightened being.
The sub-menu Buddha and Bodhisattva contains articles about the celestial Buddha(s) and Bodhisattvas