Chemotherapy in general is mostly just poison. The general idea being that your body is more capable of bouncing back from damage than mutated cancer cells that have a runaway growth problem.
It's a bit more than that. Cancerous cells will tend to absorb more chemotherapy drugs than non-cancerous cells do, because rapid cell division means that they're absorbing more of everything.
"slash, poison and burn" for surgery, chemotherapy and radiation therapy - a phrase from a well know cancer doctor mentioned on NPR couple days ago.
As another commenter mentioned it isn't just a poison though. Just for example - the artemisinin - the anti-malaria drug that was discussed recently being the subject of the recent Nobel prize - is looking promising so far as a [low toxicity] chemotherapy drug due to the same super-oxide interaction with iron-rich cells that makes it an anti-malaria drug as cancer cells are iron-rich too.
