All states have at least one; larger states have several, divided by geographic district within the state (e.g., the Western District of Pennsylvania).
Many issues before federal courts deal with application and interpretation of federal legislation. Often a circuit split can occur, meaning the circuits diverge in matters of their interpretation. Circuits frequently look to each other’s opinions as persuasive authority and often consensus will emerge; if not, the U.S. Supreme Court may grant certiorari to resolve a split in the circuits.
Federal courts may hear cases on state law issues under diversity jurisdiction. In such cases, the federal courts look to opinions of the highest court of the state as mandatory authority.