What is cancer ?

...... Defining Cancer ......

Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues.
Cancer cells can spread to other parts of the body through the blood and lymph systems.
Cancer is not just one disease but many diseases.
There are more than 100 different types of cancer. Most cancers are named for the organ or type of cell in which they start - for example, cancer that begins in the colon is called colon cancer; cancer that begins in basal cells of the skin is called basal cell carcinoma.
Cancer types can be grouped into broader categories.
The main categories of cancer include:
Carcinoma - cancer that begins in the skin or in tissues that line or cover internal organs.
Sarcoma - cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
Leukemia - cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of abnormal blood cells to be produced and enter the blood.
Lymphoma and myeloma - cancers that begin in the cells of the immune system.
Central nervous system cancers - cancers that begin in the tissues of the brain and spinal cord.
(For definitions of other cancer-related terms, see NCI's Dictionary of Cancer Terms.)

Origins of Cancer
All cancers begin in cells, the body's basic unit of life.
To understand cancer, it's helpful to know what happens when normal cells become cancer cells.
The body is made up of many types of cells.
These cells grow and divide in a controlled way to produce more cells as they are needed to keep the body healthy.
When cells become old or damaged, they die and are replaced with new cells.
However, sometimes this orderly process goes wrong.
The genetic material (DNA) of a cell can become damaged or changed, producing mutations that affect normal cell growth and division.
When this happens, cells do not die when they should and new cells form when the body does not need them.
The extra cells may form a mass of tissue called a tumor.
(Image from Understanding Cancer Series: Cancer.)
Not all tumors are cancerous; tumors can be benign or malignant.
Benign tumors aren't cancerous.
They can often be removed, and, in most cases, they do not come back.
Cells in benign tumors do not spread to other parts of the body.
Malignant tumors are cancerous.
Cells in these tumors can invade nearby tissues and spread to other parts of the body.
The spread of cancer from one part of the body to another is called metastasis.
Some cancers do not form tumors.
For example, leukemia is a cancer of the bone marrow and blood.

Additional Information
Find a type of cancer:
A to Z List of Cancers
Cancers by Body Location/System

More cancer topics:
Cancer Prevention
Cancer Genetics
Cancer Causes and Risk Factors
Screening and Testing to Detect Cancer
Cancer Treatment
Coping with Cancer
Cancer Statistics
Clinical Trials
Cancer Publications
The risk of developing many types of cancer can be reduced by practicing healthy lifestyle habits, such as eating a healthy diet, getting regular exercise, and not smoking. Also, the sooner a cancer is found and treatment begins, the better the chances are that the treatment will be successful.

Note : Information from National Cancer Institute ( U.S. National Institutes of Health )
http://www.cancer.gov/cancertopics/what-is-cancer