Prostate cancer is a type of cancer that occurs in the prostate gland of a man. The prostate is a walnut-sized gland of the male reproductive system that lies just below the bladder, surrounding the urethra. The prostate gland produces the seminal fluid that helps nourish and transport sperm during ejaculation. When the cells in the prostate gland start dividing uncontrollably, it forms a tumor. This eventually leads to prostate cancer.