A visa is an official endorsement on a document or passport denoting that the bearer may enter and/or exit the country or territory which issued it. Visas usually have conditions attached which relate to the purpose that its holder has for going to the country concerned. For example a visitor's visa holder will usually be prohibited from working or studying in the issuing country or territory.
In fact, many countries have a list of countries and territories whose citizens do not need visas to visit if their stay is within a certain time limit. However, people who wish to work, study or immigrate normally need to apply for a visa to do so beforehand. The main exception to this is for European Union citizens seeking to exercise treaty rights in another member state.