Cancer is the name commonly used to describe a malignant disease, which may occur in any part of the body. Cancer generally begins with damage in DNA of human being. DNA is like a set of instructions for human cells, telling them how to grow and divide. 

Normal cells often develop damages in their DNA, but cells have the ability to repair most of the damage. When they are unable the repair the damage, the corrupted cells often die. Certain mutations (damages), which remained, uncured, causing the infected cells to grow and results cancerous. Mutations also make cancer cells to live more than a normal cell's life span. This causes the cancerous cells to increase in their volume where as the normal cells have the particular life span after which they die automatically because of this the volume of normal cells remain controlled but due to mutations the cancerous stay alive more than normal cell's life span and also misbalance of presence of cells in body and cause many deficiencies in the body which result as various types of cancer according to the body part in which they are present.

Because cancer cells continue to grow and divide, they are different from normal cells. Instead of dying, they outlive normal cells and continue to form new abnormal cells.
Damage in DNA helps the development of Cancer cells. DNA is a substance present in every cell, which controls and directs the cells for all activities. Mostly when DNA damage the body is able to repair it. But in the case of cancer cells, the damaged DNA is not repaired. People can inherit damaged DNA, which accounts for inherited cancers. A person's DNA becomes damaged by exposure to something in the environment, like smoking. 
According to Scientists some cancer are caused by genetic factors, while other forms are caused by environmental conditions. Curing cancer has been a major goal of medical researchers for decades, but development of new treatments takes time and money. Already there are many forms of cancer, which are no longer considered untreatable. Some cancers, such as leukemia (blood cancer), can actually stop growing as suddenly as they started. This is called remission, and is often seen as a tremendous blessing for patients who have fought their conditions for years. 

There are over 100 different cancers. Cancer develops when cells begin to grow and behave in an abnormal way. The cells start to multiply excessively and form a lump or mass (a tumour).

the general causes of cancer are Tobacco (smoking, or the use of tobacco in any alternative way), Ultra Violet Radiation, Industrial chemicals, Viruses, Genetics 

Science may yet find the root causes of all cancers and develop safer methods for shutting them down before they have a chance to grow or spread.