Portugal is part of the Schengen area, which comprises 29 countries that have chosen to allow the free movement of citizens without border controls. Those countries are Austria, Belgium, Bulgaria, Croatia, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden and Switzerland. Countries in the Schengen area apply a common visa policy for short stays.
Annexes to Regulation (EU) 2018/1806 of the European Parliament and of the Council of 14 November 2018 list the countries whose nationals need a visa to enter the Schengen area and those whose nationals are exempt.
The European Union's External Action services has issued an FAQ document which should also answer most queries such as the single application procedure in case of travel in multiple Schengen countries. It is accessible on the following webpage: https://eeas.europa.eu/sites/eeas/files/frequently_asked_questions_en.pdf