A passport is an official travel document issued by your home country. It proves your identity and citizenship and is generally required for international travel and for re-entering your own country.
A visa is permission issued by a destination country (often through its embassy or consulate) that lets you enter and stay there for a specific purpose and length of time. It is frequently a stamp or sticker placed in your passport, though some visas are electronic.
In short: you almost always need a passport to travel abroad; you need a visa only when the destination requires one for your nationality and trip purpose. Many itineraries need both.
Source: Travel Visa Vs. Passport: Key Differences — Capital One