The U.S. passport identifies the bearer as a U.S. citizen or national. It is a request to foreign governments to permit travel or temporary residence in their territories and access to all lawful, local aid and protection. It also allows bearers access to U.S. consular services and assistance while abroad and re-entry into the U.S.
A travel visa is an official government document that temporarily authorizes you to be in the country you are visiting. Many countries require a visa to gain entry. The visa usually is in the form of a stamp or sticker placed IN the passport and is obtained from the country you are visiting.