To ensure a separation of powers, we have three branches of government: the legislative, executive, and judicial branches. The legislative, executive, and judicial branches create laws, presidential proclamations and executive orders, and case law, respectively. Administrative agencies, also under the executive branch, can create regulations.
The three branches of government give us our basic sources of primary law: statutes, judicial opinions, regulations, and executive orders/proclamations.
This is the basic structure of the U.S. federal and state court systems:
The U.S. Supreme Court is the highest court in the land, followed by intermediate Courts of Appeal (or U.S. Circuit Courts), and District Courts, also called “trial courts", at the lowest level. State court structures generally follow this pyramid hierarchy, but can vary by jurisdiction. There’s usually a State Supreme or Superior Court at the top, followed by an intermediate appeals court and trial courts at the bottom.
The geographic boundaries of the U.S. Courts of Appeal and U.S. District Courts are broken up into 12 different regions, and each region has one circuit court. Eleven of those regions are numbered, and the 12th is the District of Columbia Circuit. This map displays the geographic boundaries.