Cancer does not have any comprehensive definition. In the modern scientific world cancer is understood as a group of disease characterized by uncontrolled growth and spread of abnormal cells. If the spread is not controlled, it can result in death.

Cancers are classified as solid tumors or hematological malignancies. Solid tumors are initially confined to specific tissue or organ sites. Hematological disorders on the other hand involve the blood and lymph system and consequently, are usually disseminated throughout the body at the time of diagnosis. Solid tumor is often described as slow growing in relation to the fast-growing cancers of the hematological malignancies.

Benign tumors remain confined to their site of origin. Malignant tumors, by contrast, can escape their site of origin, invade surrounding tissue, and enter the blood stream and lymphatic system and - having spread to distant site of the body - set off secondary growths. These secondary growths are known as metastases. It is cancer in this metastasized or disseminated form that is so lethal.