A more thorough explanation:

Overview: The preemption doctrine refers to the idea that a higher authority of law will displace the law of a lower authority of law when the two authorities come into conflict.

Federal Preemption: When state law and federal law conflict, federal law displaces, or preempts, state law, due to the Supremacy Clause of the Constitution. For example, the Voting Rights Act, an act of Congress, preempts state constitutions, and FDA regulations may preempt state court judgments in cases involving prescription drugs.

State Preemption: Similar to federal and state laws, state laws will usually prevail when state and local laws are in conflict. Typically, there are three types of conflicts regarding preemption: outright conflict, express preemption, and implied preemption. Outright conflict occurs when an ordinance directly opposes a state law. Express preemption occurs when the state law directly opposes a local power. Implied preemption occurs when the local ordinance prohibits an act permitted by the state legislature, when a local ordinance permits an act prohibited by the state legislature, or when there is clear legislative intent that the "field" is preempted by state law.

Examples: Congress has preempted state regulation in many areas, such as medical devices. In some cases, Congress preempted all state regulation. In others, such as labels on prescription drugs, Congress allowed federal regulatory agencies to set national minimum standards, but did not preempt state regulations imposing more stringent standards than those imposed by federal regulators.

Explanation: The examples illustrate how federal preemption works and how Congress can preempt state regulation in certain areas. The examples also show how state preemption works and the different types of conflicts that can arise between state and local laws.