Chemotherapy is used to describe medications that treat cancer. In order to understand how these medications work, we need to understand a bit about tumor cells. Tumors are made up of cells that are reproducing at abnormally high rates. Normal cells know to stop reproducing (or dividing) when they come into contact with other cells. In the case of a tumor, this stop mechanism is missing, causing cells to continue to divide over and over. The RNA or DNA of a cell tell it how to replicate itself, and classic chemotherapy (which excludes immunotherpeutics and biological response modifiers) works by destroying this RNA or DNA. The more rapidly the tumor cells are replicating, the better chemotherapy is able to kill the cells.
Cell replication occurs in a series of phases, called the cell cycle. The cell cycle phases are: resting (G0; nothing is happening), G1 (or gap 1; a growth phase), S (synthesis; the replication of DNA occurs), G2 (gap 2; another growth phase), and M (mitosis; the actual division from 1 cell into 2). (See graphic of cell cycle)