Testicular cancer is a type of cancer that occurs in the testicles, which are part of the male reproductive system. Testicular cancer usually starts in the cells that make sperm, called germ cells. There are different types of testicular cancer, with germ cell tumors being the most common. These can be divided into seminomas and non-seminomas. Seminomas generally grow more slowly and respond well to treatment, while non-seminomas tend to grow more quickly and may require more intensive treatment.