Cell cycle regulation is important for healthy growth. Internal and external factors can regulate cell division.
External factors include physical and chemical signals. One example of a physical factor is cell-to-cell contact. Most mammal cells will stop diving once they touch other cells. An example of chemical signals would be growth factors which are proteins that stimulate cell division.
Kinase and cyclin are the two most important internal cell cycle regulators. The Khan Academy video, Cell Cycle Control, has a great explanation of kinases and cyclins.