well, it all depends on what viewpoint ur coming from in my opinion. to me, buddha was just a person, just like u and me. however, he was enlightened, which means that he understood the true nature of reality and the world around us. now, we all can become enlightened, just like the Buddha, and he taught us how through his words which have since been compiled into what is know as the pali cannon. but, the pali cannon explains how the Buddha became enlightened and the path that lead him to enlightenment, as well as some other things. also, im assuming that by 'Buddha' that u mean Siddhartha Gautama. if you are asking what is a buddha, that is someone who is fully enlightened. it also depends on what sect of buddhism that u practice as to how many buddhas u believe have existed. this is my understanding of buddhism from my own studies, so if anyone finds any faults in my comments, please inform me so that i can correct it. i hope that i have answered your question and if u have any more, please feel free to ask.
be well in peace