Travelers wishing to visit a foreign country for pleasure are required to have a tourist visa in addition to their passport (unless the country is under some sort of visa waiver program). A visa is official document that allows a traveler into the country. Visa applications are done through the embassy or consulate of the country the traveler wishes to visit. In an application for a tourist visa, the traveler will need to show proof of funds (proving that he has no intention of working in that country) and proof that the traveler intends to leave the country.
It is up to the traveler to get a visa. If a person fails to have a visa, he will be turned away at the airport and won't be allowed to board his flight. You need to give yourself plenty of time to get your tourist visa application approved. Times will vary depending on the embassy where you send the application to. You may even be asked to attend an interview at the local embassy. It can take as long as two months to get a tourist visa approved.
While visas are usually only valid for one country, certain groups of countries have come together to develop common visas. With these common visas, a tourist visa is issued for entry into not just one country, but all the member countries. The most well-known common visa is the Schengen visa in Europe. Visitors that are non-EU or non-EEA citizens may apply for the Schengen visa which allows them travel throughout all of the countries in the Schengen Agreement.
For tourists to Australia, rather than a visa, a traveler may be able to obtain an electronic travel authority (ETA). The ETA is only valid for passport holders of certain countries (44 countries currently are eligible). Instead of applying for a visa, for a small fee a traveler can go online and fill out a form. The ETA is usually granted immediately. Rather than getting a stamp in your passport or paper documentation, your passport number is linked to an ETA number in the immigration computer system. Essentially, it is an electronic visa.
Another program that tourists may come across is the visa waiver program. This program was created by the United States government. In this program, tourists to the United States that are citizens of one of the visa waiver program's member countries and that plan to stay for less than 90 days, can enter the U.S. without a visa. The program is reciprocal, and so U.S. citizens can in turn visit those member countries under the same program. Recent members to the program are the Czech Republic and South Korea.
