(i) Benign tumours are the masses of cells which remain confined to their original location and do not spread to other parts of the body and cause little damage.

(ii) Malignant tumours are the masses of proliferating cells called neoplastic or tumour cells. These grow very rapidly, invading and damaging the surrounding normal tissues.

(iii) Transformation of normal cells into cancerous, neoplastic cells may be induced by physical, chemical or biological agents. These agents are called carcinogens. For example X-rays, gamma rays, UV radiations and some chemicals like EtBr.

(iv) The genes which may lead to oncogenic transformations of the cells are called oncogenes.

(v) Contact inhibition—Whenever normal cells come in contact with each other, after a definite time they inhibit each other’s excess growth and multiplication. This property of normal cells is called contact inhibition which maintains the normal shape and size of the body. But cancer cells appear to have lost this property which results in their uncontrolled growth and multiplication.