Basically, sand, water, salt, and oil fall down and you can mess with them with things like walls, fire, wax, and plants. There are also other versions, like "Hell of Sand," which has zombies, and "Pyro Sand," which has more explosive items.
There are also mods, like "Burning Sand," which has many more elements, and (in my opinion) the best interface. Another, wxSand, has a good interface too, but not as many elements as some of the others. My favorite is EnigmaSand because you can use XML to make new elements and change how they interact.
Links:
- FSG Wiki: http://www.fallingsandgame.com/wiki/index.php/Main_Page
- Official site/forums: http://www.fallingsandgame.com/
- Burning Sand official site: http://www.naglmax.de/
- EnigmaSand download (I like 3-Alpha): http://www.fallingsandgame.com/viewtopic.php?t=696
- wxSand site: http://www.piettes.com/fallingsandgame/index.html
Edit: Topic description, missing space

