Cigarette smoke consists of a mixture containing thousands of different organic chemicals.
They act on the DNA of the cell and change the genetic code within genes. A normal cell does not multiply unless there is a good reason for it to do so. When it does multiply, it stops when it receives an order from the body to do so. It does not harm it's local environment, and it does not stray into other parts of the body. This is because the normal cell has genes that represent a sort of computer program or blueprint specifying the proper behavior of the cell.