Manchester United is a storied soccer (football) team in the UK, with lots of championships and history and several players who have famously cheated on their WAGs. But enough about that. They play in the Barclays Premier League and though they aren't having the greatest season (just 2nd place!), they're still one of the best teams in the league.
Crystal Palace (which for a couple of years had a developmental team here in Maryland, which unfortunately failed in a rather ugly fashion) plays in the division below the Premier League. The leagues are kind of fun; every season the bottom teams in the Premier League get relegated to the lower division, and the top teams in the lower division move up.
So Crystal Palace beating Man U. is kinda like a Division 1-AA football team beating Alabama this year. Really unlikely, even if Man U. used its reserves and not its experienced vets in a Carling Cup quarterfinal.
And that's what they did, aided by an ESPN Top 10 Highlight goal.
Match report from the Telegraph UK