The difference between football and American football.

American football is called American football. In order to distinguish football (unlike football, China has a quarterback), it is called American football.

By the way, rugby is called rugby, and the simplest difference from American football is that American football wears armor, while soccer is just a general jersey (so I personally like rugby and don't want to watch American football).

In the United States, football is called soccer, and what Americans call football refers to American football.

In British English, football is what we usually call football.