Good question. Tumours can be benign or malignant. Benign tumours aren’t cancer and won’t spread to other parts of the body, and generally don’t do much harm unless they become big enough to interfere with some important function. Malignant tumours, on the other hand, are cancerous and very dangerous. Sometimes a benign tumour can turn into a malignant one, but this is quite rare.
Now, not all cancers require tumours. For example, leukaemia is caused my uncontrolled growth of cells in the blood, and these cells don’t aggregate together as a tumour.
Cancer is the term we use in general for any disease in which cells are growing and dividing uncontrollably and invading normal tissues of the body.
A tumour is a lump of abnormal cells. It can be cancerous (“malignant”), if it’s growing rapidly and invading other tissues, and perhaps even spreading to other parts of the body. Or it can be non-cancerous (“benign”) if it just sits there.