During puberty, many hormones are made by the body. Along with androgens (male hormones), some amount of estrogen (female hormone) is also produced. While the hormones are sorting themselves out, the boys start developing some breast tissue during the process. When the breast tissue is so large that it starts to be visible, it is referred to as gynecomastia. I have found an excellent article regarding the gynecomastia like its causes, symptoms, and treatment entitled " Breast Development in Boys – Causes, Symptoms, Prevention and Treatment" at zovon.