The game involves four main falling particles: sand, water, salt, and oil. Each of these particles have special properties that can be manipulated; among these include burning, desiccating, growing, eroding, and more.
Along with these four, main particles are auxiliary environmental manipulators: Wall, Fire, Plant, Spout, Cera (or wax), ???, and Erase. By putting these together, one can thoroughly enjoy the modeling and the construction of very complex structures and systems.
More information about falling sand is available here.
Several spin offs of the game exist, and they are available here.