What is cancer?
Cancer is not one disease but a large group of related conditions. In all of them, some of the body's cells begin to grow out of control. This is a general explanation; for anything about your own health, please see a doctor.
How it starts, in simple terms
The body is made of trillions of cells that normally grow, divide and die in an orderly way. When cells grow old or become damaged, they die and new cells take their place. Cancer begins when this orderly process breaks down and abnormal or damaged cells keep growing and multiplying when they should not. Over time these cells can form a growth, and some can spread to other parts of the body (National Cancer Institute).
Why it happens: changes in genes
The US National Cancer Institute (NCI) describes cancer as a genetic disease: it is caused by changes to genes that control how cells grow and divide. According to NCI, these changes can come from errors that happen as cells divide, from DNA damage caused by harmful exposures such as tobacco smoke and ultraviolet rays from the sun, or, less often, they can be inherited from a parent. NCI also notes that the body normally removes cells with damaged DNA, but that this ability goes down with age, which is part of the reason cancer risk is higher later in life.
The World Health Organization (WHO) similarly describes cancer as developing in a multi-stage process, often from a pre-cancerous change to a malignant tumour, through an interaction between a person's genetic factors and outside agents (WHO cancer fact sheet). If you want to know more about the inherited part, see is cancer hereditary?
Benign and malignant: what the difference means
A tumour is a lump of tissue formed by extra cells. NCI explains that tumours can be cancerous (malignant) or not cancerous (benign):
- Malignant tumours can grow into nearby tissue and travel to distant places in the body to form new tumours.
- Benign tumours do not invade nearby tissue or spread. When removed they usually do not grow back. They can still sometimes be large or cause serious problems, for example in the brain, so they still need a doctor's assessment.
Not every cancer forms a lump. NCI notes that cancers of the blood, such as leukaemias, generally do not form solid tumours.
Key terms you may hear
- Tumour: a lump or mass formed by extra cells. Not all tumours are cancer.
- Benign: a growth that does not spread to other parts of the body.
- Malignant: a growth that can invade nearby tissue and spread; this is what "cancerous" means.
- Metastasis: when cancer cells spread from where they started to other areas. A cancer that has spread keeps the name of the place it started; for example, breast cancer that spreads to the lung is still called breast cancer (NCI). See what is metastatic cancer?
- Carcinoma in situ: abnormal cells that have not invaded nearby tissue. NCI notes it is sometimes called stage 0 and is usually treated because it may become cancer.
- Stage and grade: stage describes how large a cancer is and how far it has spread; grade describes how abnormal the cells look under a microscope. Both are explained in cancer stages explained.
Types of cancer
NCI counts more than 100 types of cancer. They are usually named after the organ or tissue where they start, and sometimes after the type of cell involved. Our overview of common types of cancer explains the main groups, such as carcinomas, sarcomas, leukaemias and lymphomas.
Learn about screening, reducing risk, and signs to discuss with a doctor.
Sources
Last reviewed: 15 October 2026.
Common questions
Is every tumour cancer?
No. Many tumours are benign, meaning they do not invade nearby tissue or spread. Only a healthcare professional can determine what a growth is, using appropriate tests.
Can cancer be cured?
Outcomes vary enormously by type, stage and individual circumstances. The World Health Organization notes that many cancers can be cured if detected early and treated effectively. Your medical team is the right source for your situation.
Is cancer caused by something I did?
Usually it is not possible to know exactly why one person develops cancer and another does not. NCI explains that many cancer-causing gene changes come from random errors as cells divide. Cancer is never anyone's fault.