Under normal conditions, the cells in the human body divide and grow in an orderly, controlled manner. Cancer occurs when cells become abnormal and rapidly form more cells in an uncontrolled way, forming a mass of tissue. This mass of tissue is called a tumor, or lesion.
Tumors can either be malignant or benign. Malignant tumors can invade surrounding tissue. Cells from malignant tumors can also break away, usually by way of the bloodstream, and travel to other parts of the body, where new tumors may form. This is how cancer spreads. The term for this is metastasis. A tumor that forms in this way is always made up of cells like those of the original site.
When a tumor is not malignant, it is called a benign tumor. Benign tumors can often be removed with surgery and usually do not recur. The cells in benign tumors do not invade other tissues or spread to other parts of the body. In other words, these tumors do not typically metastasize.