A civil war is a war in which the parties are segments of the same country or empire. The contention is often over matters of religion, ethnicity or politics, but is sometimes simply a struggle for power between two or more rival groups.
There were several major civil wars in the Roman Empire. The United States Civil War was fought ostensibly over the rights of the individual states versus the power of the federal government, but the cause of dispute over which those rights were claimed was the institution of slavery.
Many revolutions were in fact civil wars. If the rebelling party was successful in overthrowing the established government, it is usually termed a revolution, but if they were not successful it is usually termed a civil war. Famous revolutions include the American Revolution, the French Revolution and the Irish Revolution.