What is the difference between anti-platelet drug and anticoagulant?

Different Actions. Anticoagulants are medicines which keep the blood from clotting by action on factors in the blood which are needed to stop bleeding. Oral examples would include warfarin and a new agent pradaxa. Anti-platelet medications interfer with platelet function making them less "sticky" hence less likely to cause a thrombus. Oral examples would include aspirin, plavix, (clopidogrel) and effient.
Good question. Clotting involves a chain-reaction of biologically active proteins in the body. This "coagulation cascade" culminates in formation of fibrin, a substance that is the bulk of clots. However, clots are also comprised of platelets, a type of blood cell that adheres to the fibrin. Anticoagulants specifically inhibit steps in the chain-reaction of proteins, whereas anti-platelets inhibit these cells.