Although there are many kinds of testicular cancer, these kinds are usually placed in two categories: seminoma and nonseminoma. Both are known as germ cell cancers because they begin in germ cells. Germ cells are cells that have the potential to grow and develop into large numbers of different tissues and organs in a developing fetus. Forty-percent of testicular cancers are of the seminoma type. Seminomas tend to respond well and quickly to treatment. There are several different types of nonseminomas. Nonseminomas are faster growing that seminomas and have a tendency to spread to other parts of the body. There are other kinds of testicular cancer, but they are very rare.