Viewed historically, Zen is a form of Mahayana Buddhism that evolved from the original forms Buddhism that were established in India after the Buddha's death around 2,500 BC. Many aspects of original Buddhism are retained in Zen, including respect for Shakyamuni Buddha and his teachings. However, the degree of transformation Buddhism underwent when it took root in China and evolved into Chan (later called "Zen" in Japan) is difficult to overestimate, resulting in a path of radical nondualism. Both the ultimate goal of practice and the means to achieve that goal changed so radically that it's legitimate to question whether Chan is even Buddhism. If you want to walk the path of Zen/Chan, it's essential to understand how it differs from original Buddhism.