National vs International applications
National applications are those that are filed to the USPTO. They are written in order to gain a U.S. patent and therefore, must follow the rules and regulations established by the USPTO.

International applications are those that are filed in order to gain protection in countries other than the U.S. International applications can be filed under the Patent Cooperation Treaty (PCT).  

Patent Cooperation Treaty
PCT applications were set up to facilitate the idea of a single global initial application. Just imagine how complex things would be for an applicant trying to gain Intellectual Property protection for their new idea in a dozen different countries. How would they keep track of all the different filing dates let alone the paperwork?

And that's where the PCT comes in handy.  

A single PCT application may be filed to the applicant’s home country and then later, they may designate all the other countries they would like to apply for a patent in. PCT applications may only be used to apply for protection in PCT member countries. Many countries belong to the Treaty, but not all.

Before September 16, 2012 an application filed abroad may gain classification as a national application by entering the national stage. The date of the national stage is treated as the filing date of the application.

After September 16, 2012, an international application filed under the PCT in which the basic national fee has been paid may gain classification as a national application.