Cells normally divide in a controlled way, but abnormal cells keep on dividing and dividing, which forms a lump.
This cluster of abnormal cells is called a tumour. Some tumours are benign - or usually harmless - and often don't need treatment. But malignant tumours - the cancers - can spread.
Malignant tumours may be dangerous because they can invade nearby parts of the body and stop them functioning properly. Cells from malignant tumours can also break away and travel to other parts of the body, where they can form new groups of abnormal cells, called secondary growths or metastases.
What causes cancer in the first place, and how quickly the cells grow and spread, is different from person to person and from cancer to cancer. A large number of people with cancer overcome the disease, or live fulfilled lives for many years after diagnosis.