A tumor is an abnormal mass of tissue that forms when cells grow and divide more than they should or do not die when they should. Tumors can be benign (non-cancerous) or malignant (cancerous). Benign tumors typically grow slowly and do not spread to other parts of the body, while malignant tumors can invade nearby tissues and metastasize to distant organs. Tumors may develop in any part of the body and can vary widely in size, location, and behavior. Diagnosis often involves imaging studies, biopsies, and laboratory tests to determine the nature of the growth and guide treatment.