Plith is a simple puzzle game tuned to a casual level of difficulty, but with nicely rendered graphics and unobjectional music.
Each level consists of a peg board, and some initial arrangement of gears -- typically including one red one and one green one. The objective is to place additional gears on the pegs, so that the red gear will ultimately be able to turn the green gear. The difficulty in doing so is two-fold; firstly, putting a gear on some pegs will prevent a solution; and secondly, you have no control over what type of gear you will be served next. Sometimes you want a big one, and are given a small one, and have to try to find an appropriate place to put it.
Mistakes are recoverable, to some degree; you can "blow up" gears in play. But it is also possible to get yourself into an unsolvable state; luckily, each level is short, so it's easy enough to restart the level.
It is not a deep game, but has a nice feel, and is not a bad timewaster; the demo will certainly be enough for you to decide whether or not you like the game, and if so, it's cheap enough to be an impulse buy.