In order to travel to some countries, US citizens will need a tourist visa. To find out if you need one for where you’re going, simply visit the State Department website and enter the country where you’re traveling. They’ll hook you up with the specifics of visas.
If you're not a US Citizen, you should visit your embassy's website that is located in the destination to which you're traveling. They'll be able to tell you if you need a visa for your upcoming trip.