Cancer begins in the cells, which are the building blocks of your body. Normally, new cells form as you need them, replacing old cells that die. Sometimes, this process goes wrong. New cells form when you don't need them, and old cells don't die when they should. The extra cells can form a tumor. Benign tumors aren't cancer while malignant ones are. Malignant tumor cells can invade nearby tissues or break away and spread to other parts of the body.
Children can get cancer in the same parts of the body as adults, but there are differences. Childhood cancers can occur suddenly, without early symptoms, and have a high rate of cure. The most common children's cancer is leukemia. Other cancers that affect children include brain tumors, lymphoma, and soft tissue sarcoma. Symptoms and treatment depend on the cancer type and how advanced it is. Treatment may include surgery, radiation and/or chemotherapy.
NIH: National Cancer Institute