Most of the time, when someone says the Buddha, it’s in reference to the historical person who founded Buddhism. This was a man originally named Siddhartha Gautama who lived in what is now northern India and Nepal about twenty-five centuries ago.
Buddha Is Not a Name
Buddha is not a name, but a title. It is a Sanskrit word that means “a person who is awake.” What a buddha is awake to is the true nature of reality.
Who Is the Buddha? by Barbara O’Brien
I think that the article Who Was the Buddha? written by Barbara O’Brien and published on the website The Lions Roar provides one the easiest to understand explanations of who was the Buddha and what is Buddhism.
Simply put, Buddhism teaches that we all live in a fog of illusions created by mistaken perceptions and “impurities” — hate, greed, ignorance. A buddha is one who is freed from the fog. It is said that when a buddha dies he or she is not reborn but passes into the peace of Nirvana, which is not a “heaven” but a transformed state of existence.