The above link (hover over "what is cancer?" ) leads to a good description and video on the Macmillan website
The human body is made up of billions of cells. Normally, cells function for a while, then die and are replaced by new cells in an orderly fashion. This results in an appropriate number of cells that are organized by the body to perform specific functions.




Occasionally, however, cells are replaced in an uncontrolled way and are unable to be organized by the body to perform their normal function. As a result, there is an abnormal growth of cells that form a tumour. There are two kinds of tumours: malignant tumours (cancerous) and benign tumours (non cancerous).

Because of their increasing size, benign tumours squeeze surrounding parts of the body and expand into nearby areas. This can cause pain and interfere with normal function, but it is seldom life threatening.

Malignant tumours can cause pain and interfere with normal function, but they can also cause other systems in the body to act abnormally. Malignant tumours can invade nearby groups of cells or tissues crowding out and destroying normal cells.


Lymph Nodes

Cancer cells can also break away from the main or primary malignant tumour and travel to other parts of the body. The body fluids, that can carry cancer cells from the primary tumour to other parts of the body, are the blood and the lymph.

Most people know about blood and blood vessels but may not be familiar with lymph. Lymph is a nearly clear fluid that drains waste from cells. This fluid travels through vessels and into small bean-shaped structures called lymph nodes.

One function of lymph nodes is to filter unwanted substances, such as cancer cells, out of the lymph fluid. However, if there are too many cancer cells, the lymph nodes cannot remove all of them.

Isolated or disseminated tumour cells are single or small groups of tumour cells that have been separated from the primary tumour and can be found in the blood, lymph, or bone marrow. They can develop into life-threatening metastatic disease if they are untreated.