(also known under the brand names of CoumadinĀ® and MarevanĀ®) is an anticoagulant medication that is administered orally. It is used for the prophylaxis of thrombosis and embolism in many disorders. Its activity has to be monitored by frequent blood testing for the international normalized ratio (INR).
Warfarin was originally developed as a rat poison, and is still widely used as such, although warfarin-resistant rats are becoming more common.
Wikipedia knows all.