normal cells are under control but cancer cells are not.
normal cells have links and signals among them and other types of cells, but cancer is not
normal cells are subjected to apoptosis but cancer is not
Cancer cells normally have a defect in their receptor proteins on the membrane. There are several growth receptors on the surface of cells, and if one of them is faulty, cancer may form. The cancerous mass will continue to divide if there is no negative feedback from the cells surrounding it. In addtition to not being able to receive normal signals that regulate the cell cycle, cancer cells do not stop dividing when growth factors are depleted, simply put cancer cells don't require nutrients to grow, however they may still receive nutrients. As long as a cancer cell is provided with nutrients they may grow indefinately until some random point in the cycle where they may stop, rather then stopping at checkpoints in the normal cell cycle, making them somewhat "immortal". These are the most common differences in cancer cells and normal cells.
Reference: http://wiki.answers.com/Q/What_is_the_difference_between_cancer_cells_and_normal_cells