What is the difference between a passport and a visa?
A passport is issued by your own country and establishes who you are and which nationality you hold. A visa is issued by the country you wish to enter and grants permission to do so. One identifies you; the other authorises entry.
The practical consequence is that holding a passport gives you no automatic right to enter anywhere except your own country. Whether you need a visa depends on the combination of your nationality and your destination, and on the purpose and length of your visit.
Several arrangements exist. Some country pairs have visa-free agreements for short visits. Some operate visa on arrival, issued at the border. Many now use electronic travel authorisations, which are applied for online in advance, are not technically visas, and are nonetheless mandatory. Others require applying at an embassy or consulate well ahead of travel.
A visa also grants permission to travel to a port of entry rather than guaranteeing admission. The final decision rests with the border officer, and entry can be refused despite a valid visa.
Two requirements catch travellers out regularly. Many countries require a passport valid for a minimum period beyond your intended departure, commonly six months, so a passport valid on the day of travel may still be refused. And some require blank pages for stamps.
Requirements change, sometimes at short notice, and depend on your specific nationality. The authoritative source is the destination country's own immigration authority or your government's travel advice, not a general article, and it is worth checking close to departure rather than only at booking.