What is the difference between a visa and a passport?

A passport is your identity and citizenship document, issued by your own government, and it is what lets you leave and re-enter your home country and identify yourself abroad. A visa is permission granted by a foreign government to enter its territory for a specific purpose and period, tourism, business, study, work, and it is typically issued as a stamp or sticker placed inside your passport. So the passport says who you are, and the visa says a particular country has agreed to let you in.

Whether you need a visa depends entirely on your nationality and destination. US passport holders can enter many countries visa-free for short tourist stays, or with a simple electronic authorization obtained online, which is not technically a visa but functions similarly, Europe's ETIAS system and similar programs work this way. Other destinations require applying in advance through an embassy or consulate, with fees, documentation, and sometimes an interview, and some issue visas on arrival. Two things trip people up: a visa grants permission to travel to a port of entry, not a guarantee of admission, since border officers make the final call, and many countries require your passport to remain valid for six months beyond your stay. Check the destination's official requirements well ahead, since processing can take weeks.