In law, an appeal is a process for requesting a formal change to an official decision.
The specific procedures for appealing, including even whether there is a right of appeal from a particular type of decision, can vary greatly from country to country. Even within a jurisdiction, the nature of an appeal can vary greatly depending on the type of case.
An appellate court is a court that hears cases on appeal from another court. Depending on the particular legal rules that apply to each circumstance, a party to a court case who is unhappy with the result might be able to challenge that result in an appellate court on specific grounds. These grounds typically could include errors of law, fact, or procedure (in the United States, due process).
In different jurisdictions, appellate courts are also called appeals courts, courts of appeals, superior courts, or supreme courts.