What are Antipopes and how different are they from Popes?

I’ve noticed that sometimes there are people called antipopes during the papacy. Who are they and why are they there?

An antipope is someone who falsely claimed to be the Pope. Sometimes this was born out of legitimate confusion and misunderstanding, and other times it was born out of political scheming and divisions. Antipopes were not validly elected and thus were never true Popes.

Catholic Encyclopedia entry Antipope

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