Just a valid US Passport. No visa is need to enter El Salvador for US Citizens for pleasure purposes.
Every country has the right to control who is allowed to enter their country and who isn't. A visa is basically permission from the country's government to enter.
Yes you need passport with visa to enter any other country.
No, Mongolians will not need a Visa to enter Canada. They can stay in the country for up to 30 days.
Syrians don't need a visa to enter Malaysia.
The visa is not necessary for Australians citizens.
A multiple entry visa means that you can enter and exit the country (issuing the visa) as many times as you want, within the visa validity period. On the other hand, in case you were issued a single entry visa, you can only enter and exit the country (issuing the visa) once. Once you have exited that country, you can not re-enter on that same visa regardless of whether the visa is valid or expired.
Yes.
No - you still need a visa to enter the UK !
Yes. In general, you'll need a visa to enter any other country, including Brazil.
You need a passport and an entry visa if you are entering Mexico from any country other then the US.
No visas are required for Americans to enter any EU country.