Cancer cells are misregulated in their divisions because they continue to divide even if there are cells crowded around them or they are not attached to a solid surface. This shows that unlike most animal cells, they don't abide by factors that control growth rates, such as density-dependent inhibition or anchorage dependence. Also, cancer cells do not heed the normal signals that regulate the cell cycle. At the various checkpoints over the cell cycle, these cells poorly self-check themselves, which allows for the cell to continue to divide excessively without end (cancer cells are "immortal"; unlike most animal cells that stop after a certain number of divisions) and invade other parts of the body. This could be due to mutations in the various proteins, such as cyclins, Cdk's, and receptors, which are responsible for the normal control of the cell cycle.
Work Step by Step
To understand this answer go through the material on the cell cycle, paying close attention to the checkpoints that use growth factors and proteins to determine whether or not cells should replicate. The uncontrolled cell division seen in cancer cells are the result of malfunctions occurring at these checkpoints.