A visa is a document which grants the holder of the visa the right to enter into the country which issued the visa. Visas may take many of a number of forms, including full paper documents, as mentioned above, and stamps in passports.
Visas are not always necessary for travel to all countries, as in some instances, nations might have relationships which allow citizens from one nation to travel to the other without a visa, and vice versa. Furthermore, in some forms of travel, visas may be necessary, while others may not requires a visa. For example, traveling to Canada by driving across the Canadian-American border may not require a visa, while flying to a European nation might still require a visa.
A visa is normally granted to a holder for a particular purpose, depending upon the reason for which the holder has said that he or she is planning on traveling to the nation in question. For example, someone who is traveling to a country for the sake of recreation or tourism might apply for a tourist visa to that country, as opposed to someone who might be traveling to the same country for the sake of business, who might then apply for a business visa.
Visas might be particularly important for those individuals who plan on traveling to a country in order to immigrate there, as visas can dictate the amount of time that these individuals may remain in the country, and thus determine how long they have to apply for some form of permanent residency identification.