The most important document to have when traveling is a passport. The U.S. government requires a passport or other document to exit the country, and requires only a passport for re-entry. Additional documents may be required depending on the destination and duration of travel.
In much of Western Europe, American and Canadian citizens require only a passport, as long as their visit is less than six months and is not intended for work. People planning on longer stays or traveling for work may require a visa, which is a stamp placed in the passport by a government that gives the bearer permission to travel in that country. Other helpful documents include an International Driving Permit, pre-purchased rail or plane tickets, and other identity documents.