Many Possibilities. This could be a tumor, spermatocele, lipoma, granuloma, infection, or other process. Typically, processes associated directly with the testicle are concerning for cancer and growths or findings associated with the other elements in the scrotum (epididymis) are almost always benign. An examination by your md and an ultrasound will help determine what this might be.
Scrotal mass. This may be a fluid filled cyst. You should have this evaluated by a urologist. It is not likely a tumor..
Testicular/epididyma. All young men should be performing regular self-examinations weekly (much like women should perform regular breast self-examinations. If you have two testicles, then you should compare one to the other - any discrepancy should be evaluated by a urologist since testicular cancer is the most common malignancy in young men.
Spermtocele. If it's located behind or above the testicle, it's most likely a spermatocele or epididymal cyst. They are benign but may require removal if they are painful or large.