It sounds really simple, but it's quite a lot of fun. It's a really eccentric Japanese game that's sort of gained a cult following. That's why they did a sequel.
Backstory: Well, the King of the Cosmos and the Universe sends you to Earth to make Katamari's. You are his son, the prince, who accidentally broke the sky one day. Suddenly all the stars are gone. So, to rebuild the night sky, the prince is sent to Earth to collect things, to make Katamari's (which are basically big balls of stuff). So you're rolling up stuff on earth. You start off rolling up matches and sticks of gum... there are objectives and time limits and size qualifications of course. Once you finish a Katamari (in each objective phase) it becomes a star in the sky. As the game progresses, you create constellations, the moon, the sun and so on. Also, you roll up larger objects. You roll up people and buildings and trees even clouds and mountains. It's a lot of fun. You find little hidden prizes on Earth for the Prince. You can play multi-player with the cousins. The dialog between the King and Prince is absolutely hilarious.
I also have We Love Katamari listed. Which is the sequel. It has more specific objectives, and many say is a better game. You do things like roll up as many animals as possible in a few minutes, make saturn's rings and so on.