Water polo, or Water ball, is a team water sport. The playing team consists of six field players and one goalkeeper. The winner of the game is the team that scores the most goals. Game play involves swimming, treading water (using a sort of kicking motion known as “eggbeater kick“), players passing the ball while being defended by opponents, and scoring by throwing the ball into a net defended by a goalie. ‘Man-up’ (or ‘power play’) situations occur frequently. Water polo, therefore, has strong similarities to the land-based game of team handball.
You can learn more about the game on Wikipedia, watch an example game here, and if you want the rules they are here in PDF format on the FINA website. There’s no need to learn all the rules to start playing, just come down on Sunday evenings and we’ll teach you.
Here’s a fun video on the basics.