When traveling to the United States as a foreign citizen a visa must be provided. The US State department has a link on their website to provide information. Generally foreign citizens must obtain a nonimmigrant visa for temporary stay or an immigrant visa for permanent residence. Please refer to the website reference material to decide which type of visa is required.
For United States citizens traveling abroad you may need a visa issued by the embassy of the country you are travling to. Further information is provided in the link for Americans Traveling Abroad.
If you need additional information then check out these resources to find out who/when a visa is needed.
This is another great website that gives you information about visas and it’s for all Nations.
This site is aweome and provides you information about visa requirements as well as health information.