Cancer transforms normal cells into cancer cells that then form a malignant tumor and also "metastasize" to form new tumors at other locations within the body.
From Cell to Cell 
The first cancer cell in someone's body started out as a seemingly normal cell that one day began mutating into a malignant cell -- over a period of several generations of cell division.
Once there are full-fledged cancer cells, they begin multiplying much more rapidly than normal cells - as they divide in a haphazard manner. As a result, these cancer cells often pile up into a non-structured mass referred to as a malignant "tumor."
From Place to Place
A malignant tumor can destroy the part of the body where it originated (the"primary site") and then spread to other parts of the body where new tumors are created and even more destruction occurs. When a cancer moves from place to place like this, it is referred to as "metastasizing."
Staging
Cancer patients often ask their doctor: "How advanced is my cancer?" The disease usually is described as being at one of four stages. For example, "Stage 1" means there is evidence of a tumor but only in the tissue of origin, whereas "Stage 4" means the cancer has metastasized to a secondary site, distant from the primary site where it first originated.