Cancer is not just one disease but a collection of unique conditions in which cells in the body become abnormal then grow and multiply out of control. Although this is true of all cancer types, whether it is breast cancer, leukemia, lymphoma, lung cancer, or any of several other types, each cancer has unique characteristics and is treated with a unique set of therapies.

In some types of cancers, when cells grow out of control they usually form a mass, called a tumour. These tumours not only enlarge locally, but also have the potential to invade and destroy the normal tissue around them and to spread to distant parts of the body. These tumours are called malignant tumours or cancers.

Some tumours grow and enlarge only at the site where they begin. These tumours are not cancerous, and are referred to as benign tumours.

Distant spread of cancer occurs when malignant cells become detached from the original (primary) tumour, get carried to other parts of the body through the blood or lymphatic vessels, and establish themselves in the new site as an independent (secondary) cancer. A tumour that has spread in this manner is said to have metastasized. The secondary tumour (or tumours) is called a metastasis (or metastases).

Since cancer may develop in any tissue of any organ, there are many different types of cancer.