In mainstream America, at least:

-Your first name is what people call you by.

-Pretty much everybody has middle names but most people don't use them outside of legal paperwork. Often people will give their kids middle names to honor older relatives' first names. You can also just pick something that sounds good with the kid's first name. Having more than one middle name happens sometimes, there's no law restricting you to just one.

-Your last name is your family name. Usually it comes from your father or husband, but there is no law for choosing last names, so there are a lot of alternatives. Some people, especially those of latin american descent, will give their kids two last names.