A lung cancer cell going thru division. Credit: NIH

Each cell in the human body must follow certain control processes, rules, and regulations as it goes from birth, growth, then to death. Whenever these mechanisms fail to control the cell’s growth, which is due to most cases some genetic damage to the genes—this marks the beginning of the cell going from normal to abnormal or mutated and starts the onset of cancer. After the mutation takes place, the cell begins to multiply aggressively and at a faster rate than normal. With the breakdown of the control mechanism, the cells do not go thru the cycle of cell death or apoptosis.
Since the human body is made of trillions of cells, cancer can start anywhere. Many cancers will form a solid mass or tumor, others do not. For example, breast cancer most cases will form a tumor, whereas blood cancer or leukemia will generally not form the tumor.
Cancers that have formed into tumors are malignant—means they can invade or spread to nearby groups of cells or tissues. On top of that, in these tumors, some cells can break off and migrate to a distant place in the body via blood or lymph nodes. Then form a new tumor far from the original location of cancer. This is known as metastasis.
Tumors or cancers that do not spread or invade or spread to other tissues or organs is called the benign tumor. These benign tumors usually don’t regenerate after removal, whereas malignant tumors sometimes do. Most benign tumors are not life-threatening, but some can be, e.g. benign brain tumor.