Cancer ( tumor ) means a mass of abnormal cells in body.If this mass can send a cell to other parts of body to involve them , it is malignant-bad- tumor like lymphoma and if it is vise versa, it is benign-good- tumor like lipoma.
Most cells have ability to regenerate themselves and this is natural phenomenon which is occurred every day .During the cell division sometimes some cells are abnormal and  immune system as quality control section recognizes them and this action leads to  cell suicide: Apoptosis.
Sometimes immune system can’t do it’s duty properly and one of abnormal cells-cancer cell- continues its life and by division it makes a mass of  cancerous cells.

On the basis of the cell’s origin, tumor  is named.

There are so many reasons which lead to tumor  like: genetic, viral infection such as leukemia in cats, radiation, hormonal imbalance, food, water , air pollution, trauma, and rarely some  medications.