Welcome to the wonderful world of Quidditch. It's easier to understand than rugby (I'm on the team by the way and it took me a half a year of bench warming before I could fully immerse myself) and a little bit more difficult than Football and seriously, I would want to play it any time.
So I will try to explain exactly what these heros on brooms are doing.
First of all understand this, it's semi-contact and everyone is on a flying broom (imagine those tackles).
Objective: You need to get as many quaffles (big brown soccerball shaped things) through the other team's hoops of which there are three. These hoops are literally three tall pols that each have a hoop at the end and you need to throw these quaffles in the hoops. Each time the ball goes trough the hoop is 10 points.
Positions:
The Keeper is kind of like the goalie except on a flying broomstick.
The Chasers are the ones that basically score the ball and try to prevent the other team from doing so.
The Beaters are the ones that carry a bat (OMG theres a bat too!) and prevent the bludgers from hitting their players. Bludgers are basically big canon ball shaped things that fly of their own accord and try to hit the players.
...and lets not forget the star of the show...
The Seeker is the one who must catch something called the Golden Snitch witch is a walnut sized ball that has wings and flies around. Once the seeker (aka Harry Potter all throughout the series) catches the snitch, the game is over. Of course catching the snitch wins 150 points but you still have to be careful that when you catch it, your team can actually win with 150 points.
Because of this, quidditch games can last days...
There is also something called the Quidditch World Cup (Book 4) that is actually a big deal for the wizarding world. It's kind of like their superbowl.
This was just the basics my friend, the threshold of the world of Quidditch there have been books written on this fictional sport, wikipedia articles http://en.wikipedia.org/wiki/Quidditch. But if you are willing to read that unfortunately long article, then you might as well read the books.
No seriously, if you are this far in this book, you might as well read the Harry Potter Series for real.
Catch the next chapter for Book 2: The Chamber of Secrets.
Please comment if there are specific things about Harry Potter that you would like to know!