Cancer cells develop from cells which are healthy, but undergo a complex transformation process.  Step one in this process is called "initiation."  This is the step in which a change in genetic material of the cell primes this cell to make it cancerous.   This change in the genetic material of the cell sometimes occurs spontaneously, and other times is brought on by a cancer-causing agent (or a "carcinogen").  Carcinogens could include several chemicals, viruses, tobacco, radiation, even sunlight (in the case of skin cancer or melanoma).  However, all cells are not equally susceptible to the influence of carcinogens.  A cell with a genetic flaw will be more susceptible.  Also, a chronic physical irritation might cause a cell to be more susceptible to any carcinogen.
 
The final step in a cancer's development is known as "promotion."  An agent which causes this promotion is known as a promoter.  A promoter could be a substance in the environment or a drug (such as a barbiturate).  Promoters, unlike carcinogens, don't work by themselves to cause cancer.  Rather, a promoter will allow cells which have undergone initiation to morph into cancerous cells.  Promotion will have no effect on a non-initiated cell.  Therefore, numerous factors, usually the combination of a carcinogen and a susceptible cell, are required to produce cancer.
 
In some cases, a carcinogen will be powerful enough to produce cancer without promotion taking place.  For instance, ionizing radiation, found in x-rays and also in nuclear power plants), may cause several forms of cancer, especially sarcomas, thyroid cancer, leukemia, and breast cancer.
 
Once a cancer has been produced, it then often has the capacity of growing directly into tissue surrounding the origin where the cancer first appeared.  It can spread to other organs and tissues, not just close to the original organ, but also throughout the body.  In some cases, the cancer will spread throughout the lymphatic system.  Spreading of this type is fairly typical with carcinomas.  For instance, breast cancer normally spreads first to lymph nodes in the vicinity.  Later, the spreading might become more extensive, reaching throughout the body.
 
Cancers might also spread via the person's blood-stream. This is typical of sarcomas.
 
A term often used to refer to spreading of cancer through the body is metastasis.  This refers to the tendency of cancer cells to break away from the primary tumor.  They then enter the blood vessels and lymphatic vessels and travel through the blood-stream.  When they reach an other organ or other tissues in the body, these tumor bits settle down and grow within those tissues, spreading the ill effects of the cancer.  Whenever a tumor cell metastasizes, we call the new tumor a metastatic or secondary tumor.  The secondary tumor, though, is comprised of cells like the ones in the original tumor. For this reason, even if breast cancer spreads to the lungs, this secondary tumor will be comprised of abnormal breast cells, and therefore it remains breast cancer, not lung cancer.