This information copied from CBSnews.com/health

Ordinarily, cells in the body grow and divide only when the body needs them, but some cells continue to divide even when the new cells are not needed. These extra cells form a mass of tissue called a growth or tumor. A tumor can either be benign or malignant. Only the malignant tumors are cancerous. Cells in these tumors are abnormal and divide without control or order. They can invade and damage nearby tissues and organs.

Invades:
When the RAGE (Receptor for Advanced Glycation End products) protein - found on the cancer cell's surface - binds with amphoterin protein, they break down neighboring tissues, enabling the cancer cell to invade. Normally these proteins play a role in early tissue development and healing, but cancer cells draft them into destructive service.

Develops
The cancer cells spread by pushing into healthy tissue. Temporary protrusions of the cell's protoplasm - called pseudopods - act as drills, probing and boring into the matrix between normal cells. Actin filamints in the cell's inner skeleton remold the cell as it crawls. Like treads, adhesion molecules grip surfaces to help the cell wriggle forward.

Spreads
The cancer cells migrate and multiply. Eventually they may enter the bloodstream and may be transported to other parts of the body, forming secondary growths.