Tumors are abnormal masses of tissue that can be solid or fluid-filled. They are not necessarily cancerous. Tumors are divided into three groups: benign tumors which are non-cancerous and cannot spread; premalignant tumors which are not yet cancerous but appear to be developing cancerous properties; and malignant tumors which are cancerous and can grow and spread. Doctors diagnose tumors through biopsies and treat them with therapies like chemotherapy, radiation therapy, targeted therapy, and immunotherapy.