Can someone explain the difference between page router and app router?

create-t3-app is using page router at the moment as far as I can tell, but the app router seems to be the latest thing. What's the difference? Should I switch? When do I switch? How do I switch? Does it even matter?
I suppose it's not that important as long as create-t3-app is still using page router?

Please excuse me if these are stupid questions lol
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