In the United States today, there are numerous sources of law. The main ones are (1) constitutions—both state and federal, (2) statutes and agency regulations, and (3) judicial decisions. In addition, chief executives (the president and the various governors) can issue executive orders that have the effect of law.
In international legal systems, sources of law include treaties (agreements between states or countries) and what is known as customary international law (usually consisting of judicial decisions from national court systems where parties from two or more nations are in a dispute).
As you might expect, these laws sometimes conflict: a state law may conflict with a federal law, or a federal law might be contrary to an international obligation. One nation’s law may provide one substantive rule, while another nation’s law may provide a different, somewhat contrary rule to apply. Not all laws, in other words, are created equal. To understand which laws have priority, it is essential to understand the relationships between the various kinds of law.