Cell division is a normal process in multi-cellular organisms. Growth and repair (replacement of dead cells) take place as a result of cell division (mitosis). Except for cells like the liver and brain cells, which rarely divide in mature adult, most cells undergo frequent division. Sometimes, however, cell division becomes very rapid and uncontrolled, leading to cancer. It should be clearly understood that rapid growth means a high rate of cell division for a particular cell type. It is possible for perfectly normal cells e.g. the blood forming cells to have a higher rate of division than some cancerous cells.
Cells which undergo rapid abnormal and uncontrolled growth at the cost of remaining cells are called neoplastic cells. The growths resulting from the division of such cells are called neoplastic growths or tumors. Tumors are commonly classified as benign and malignant. Abnormal and persistent cell division that remains localized at the spot of origin results in the so called benign tumors. Benign tumors can sometimes be fatal, e.g. brain tumors that cause pressure on vital centres. Benign tumors usually contain well differentiated cells. Tumor cells may be carried by the blood stream, or the lymphatic system, or by direct penetration to other parts of the body where they may induce secondary tumors. Such invasive cancers ultimately result in the death of the organism and are therefore said to be malignant. Malignant tumors usually contain un-differentiated cells, often with large nuclei and nucleoli.