Traveling internationally isn't frequently as simple as booking a ticket online. Many popular travel destinations need pre-registered travel visas.
For many countries, this may be quite simple, for others although the procedure and red tape may be more intimidating than obtaining your passport (which be up-to-date or your going nowhere!)
A"visa" is a temporary (or even sometimes permanent) consent to travel, reside or work inside a foreign nation. You can browse http://estavisa.com.au/ to get more info on visa services.
As US citizens, we're often privileged to have the ability to travel to a lot of states without pre-registration and this is because of political connections with that nation.
Though you might not call for a pre-registered visa to enter Italy, the individual standing next to you at the immigration line might.
Similarly, many foreign nationals are permitted entry into the United States with no visa, but a lot of them aren't.
Each nation has its own need for one another nation and should you add up the visa requirements for almost 200 nations and they are changing social and political relations with another 200 nations, things can find a bit confusing.
The US State Department maintains a fairly updated global FactBook as do the CIA and FBI. Or you could seek the experience of a Visa Expediting Service.
"Expeditors" because they're known work with overseas embassies and consulate offices to procure the right visas for their clients and maintain databases filled with requirements.