Sand is a common block found in Desert Biomes and often near water. Sand follows the laws of physics, and will therefore fall if there is no block beneath it, just like Gravel. This means that if a floating sand block, that sometimes accidently spawns during world generation, recieves a block update (has a block next to it change- change in redstone signal, or the placement/breakage of a block), all connected blocks will fall too.
Sand can be smelted into Glass, combined with Gunpowder to make TNT, or crafted into Sandstone, which is not affected by gravity in the same way sand is. As sand is the most affected block by gravity, it is the most commonly used block in making traps.