A US patent only protects your patent in the United States. You could patent something in the US and someone else could potentially copy your invention in another country and make money from selling it in another country, although they could not import it into the United States.
If you want to protect your invention internationally, you have one year to apply for patents in other countries after (submitting your patent application in the United States). You could alternatively use a Common Application Format to apply for a patent in the United States, Europe, and Japan simultaneusly. These are the three largest patent offices worldwide. The other major patent offices are the Korean Intellectual Property Office and the State Intellectual Property Office of the People's Republic of China.
You can also apply via the Patent Cooperation Treaty. The PCT includes an international phase and national phase. The international phase allows you to file a single patent application to seek patent protection in up to 148 countries. During the national phase, you would continue the patenting process with the countries where you want patent protection.
Foreign patents are also considered prior art. Prior art means that an invention has already been publically disclosed and is therefore not considered new and nonobvious. In other words, if someone has already patented your invention in another country, you would not be able to patent it in the United States.
International applications and patents can provide competitive intelligence for your field regarding what an international competitor plans to manufacture.