Testicular cancer is a disease in which malignant (cancer) cells form in the tissues of one or both testicles. The testicles are 2 egg-shaped glands located inside the scrotum (a sac of loose skin that lies directly below the penis). The testicles are held within the scrotum by the spermatic cord, which also contains the vas deferens and vessels and nerves of the testicles. 
The testicles are the male sex glands and produce testosterone and sperm. Germ cells within the testicles produce immature sperm that travel through a network of tubules (tiny tubes) and larger tubes into the epididymis (a long coiled tube next to the testicles) where the sperm mature and are stored. 
Almost all testicular cancers start in the germ cells. The two main types of testicular germ cell tumors are seminomas and nonseminomas. These 2 types grow and spread differently and are treated differently. Nonseminomas tend to grow and spread more quickly than seminomas. Seminomas are more sensitive to radiation. A testicular tumor that contains both seminoma and nonseminoma cells is treated as a nonseminoma. 
The good news is that it is one of the most treatable and survivable types of cancer. When detected early, 99% percent of guys diagnosed with testicular cancer survive it, and go on to lead normal, active lives.
According to The Testicular Cancer Foundation, the best way for men to be proactive is to do a monthly self-exam of their testicles to check for lumps, hardness or swelling. It’s easy to do in the shower.