Cancer occurs due to abnormal and uncontrolled cellular growth. This is caused by the alterations in normal cellular growth control. In normal cells, cellular growth and death are in an equilibrium. This balance is deteriorated in cancer.

As a consequence;

• Cells increase uncontrolledly
• Death of nonfunctional cells are prevented

Cell come together to form tissues, and tissues come together to form organs. When organs and tissues are forming, the cells come together in an exact order and by making common workshares. These cells are the basic units of an organism and multiply at an exact rate and under control. Besides, aging cells die at an exact rate.

The shortest description of cancer is uncontrolled increase of the cells. Structural and functional differences occur in cancer cells when compared to normal cells during this increase; cells may not function normally, or may exert novel functions which are not present normally.

Abnormally increased cells occupy the tissues and organs, and even distant organs, and will inhibit the functions of those compartments. The time from alterations of cellular control to cancer progression varies according to cancer types but it is about 15 to 20 years. Causes and mechanisms of these diseases are not exactly known. But significant progress has been observed on these issues in recent years.

Cancers are named according to the names of their original tissues and organs. Signs, symptoms and treatments are also varied according to cancer type. Most frequent sites of cancers are skin, lung, breast, and gastrointestinal and reproductive system.

The characteristics that diverts cancer cells from normal cells, and the genes that control those characteristics are important. Also, another important characteristic of a cancer cell is its ability to disseminate to another localization in the body, or to metastasis.