Can someone explain the league system to me?

Hey guys, just wondering,what are Leagues in PoE?

I'm a beginner, so should I be worried about Leagues? Should I join them? Which one? And if I do join them, can I switch Leagues or do I need to create a new character?

Can someone just shed some light on this?

Cheers.

Basically every 4 month PoE creates new leagues with different mods/mechanics/content to the game that adds something different than the regular game with a fresh start where everyone starts with nothing. New leagues are typically more populated than their parent softcore/hardcore permanent leagues. When the leagues end everything you have gets dumped into the parent league.

Its just something to keep the game fresh and interesting.