Its game about placing objects in a world and making these objects interact in sometimes logical and everyday manners, sometimes in the most irrational and insane ways possible.
I have only played the first game back in the 90s, which consisted of 2 very deferent parts.
One where you can fill the world with all the objects you need to create your own incredible machine. While very entertaining it does get a little samy, after you have made your third mouse-powered-fishbowl-crushing-machine™. However, the puzzle mode compensate for this with an insane amount of creative puzzles.
These vary from easy to stupidly hard, but even the most ridiculous puzzle never seems unfair because each object is only cable of doing 1 or 2 things.
A ball can push things around (buckets, other balls and of course cats) and will start machines if they hit their start bottom (a flash light); while a mouse trapped in a hamster wheel only power your machines if you scares it with a ball or a cat.
So even a huge system of many interacting objects do make sense after you have played this game for a while.
I can strongly recommend this game to puzzle games, but also to people who just like to play around in your own little sandbox.
On a more personal note: I know these may be big words, but in retrospect I think this game might have had a rather big influence on my life. As I today study physics, which pretty much involve pushing around with object in the same manner as this game (even thought there is a little more math involved).