Cancer cells are different from normal cells as unlike normal cells, they don't stop growing at a certain stage but keep multiplying indefinitely. A tumour is formed by billions of copies of the original cancerous cell.
The reason for this abnormal behaviour can be a hereditary mutation in a gene (but that’s only the case for 5 to 10% of cancers) or mostly caused by other factors like things people do (smoking, spending a lot of time exposed to sun without protection, bad eating habits, …). Environmental exposure to radiation or to certain chemicals have also been shown to increase the risk of cancer for the exposed persons. Finally, infections like hepatitis, HPV, and HIV are also linked to higher cancer incidence.
Because cancer cells are multiplying at a very high rate, they accumulate defects during the tumour growth, sometimes developing resistance to therapeutic agents or escape mechanisms to spread inside the body of the patient, causing metastasis.