Normal cells will not divide unless they receive outside signals that cause it to enter the cell cycle. These signals are detected outside the cell (on the cell surface) and sent into the cell. They come from three different sources; growth factors (what tells a cell to grow), cell-cell adhesion molecules (what connects one cell to another), and extracellular matrix components (what exists outside of the cell).
Normally these signals, along with other factors, help control the growth of cells, preventing them from dividing uncontrollably. Cancer cells develop the ability to grow in the absence of these external factors and no longer obey the normal regulations on cell division. They do this by, producing their own growth factors, alter their growth receptors to divide more, and even influence surrounding cells to produce growth signals. At this point cancer cells begin to behave more like independent entities living without regard for the organism as a whole.