If you've ever poked around on Newgrounds or a similar site with lots of little Flash games, you might have played one of those games where there's a tower of blocks or something and you have to knock it over by launching a cannonball or similar at it. Usually these games have a really basic physics engine for determining how the tower topples over, etc.
Here's a good example:
http://armorgames.com/play/3614/crush-the-castle
Angry Birds is just one of these. It's a little Flash game that Rovio Mobile jazzed up a little with cute graphics and sound and now charge $1 for.
It's a decent game to have if you have a smartphone. Not bad as a timekiller if you need to burn a couple of minutes when you're out somewhere. I think they've ported it to just about every current mobile phone OS in existence now so if you have one of those you can play it.