To enter Canada, an American citizen needs either a passport/passport card or a NEXUS card (a special ID card with which the pre-approved traveler can speed through the Canada-U.S. border control). Note that the passport card is only suitable for the land border. To travel to Canada from the U.S. by air, you will need a passport book.
If you are a U.S. Lawful Permanent Resident, you can enter Canada visa-free by showing your Green Card along with your national passport.