Difference between US and USA
People often use different names to refer to same country or states. For example, people use term like US (United States) and USA (Unite States of America) for America. There is no difference between the two. But there are few technical differences between United States (often refer to as States) and United States of America.
People living in USA often refer to the place as US, while the other living outside America refer to the place as USA. Even in the constitution of America, both terms are used i.e. US and USA.
United States literally refers to a group of states that are united. They are many states in the world that are united, for example, and Belgium have multiple states and can be called as united states. The term "united states" does not specifically refer to the American United States (unless it is written with capital letters, in which case it wouldn't specifically refer to the USA as there are other countries whose official names includes the phrase "United States", e.g. Estados Unidos Mexicanos, which is the official name of Mexico). However due to its frequent use and it being a top nation and world power, people refer to the United States of America as U.S. and it is most likely everyone will understand they are speaking about the United States.
This term specifically refers to the United States of America without any doubt. Mostly people use USA for formal use but not in common use, they prefer US, U.S. or "the States". But all three terms mean the same.
|Definition||Abbreviation of United States.||Abbreviation of United States of America.|
|Specificity||United States can be of other nations as well, although it's not that common.||USA is only and specifically referring to the United States of America.|
|Ease of use||US is used more commonly used among natives and non-natives.||USA is not commonly used among natives. It is only used by English speaking people, although most people know which country it is.|
|Referral term||US is used for casual reference.||USA is used for formal reference.|