There are many ways to describe who the Buddha is, according to different ways of understanding . These various interpretations have their sources in the Buddha’s teachings.
One way is to see the historical Buddha who lived 2,500 years ago as a human being who cleansed his mind of all defilements and developed all his potential. Any being who does likewise is also considered a Buddha, for there are many Buddhas, not just one.
Another way is to understand a particular Buddha or Buddhist deity as omniscient mind manifesting in a certain physical aspect in order to communicate with us.
Yet another way is to see the Buddha -- or any of the enlightened Buddhist deities -- as the appearance of the future Buddha that we will become once we properly and completely have engaged in the path to cleanse our mind of defilements and develop all our potentials.