Sand is the main landcover for Desert Biomes and Beaches. The block itself, Gravel, Anvils, the Ender Dragon egg and primed TNT are the only blocks in Minecraft that obey the laws of physics. This makes Sand dangerous when collecting it, as it may fall on a mining player and suffocate them. This block is one of the main reasons to never dig straight up when mining.
Sand can generate over a water pool, or make a floating structure. Once a block update (breaking or placing a block) occurs in a block adjacent to the sand, all the sand will fall. This also applies to Gravel but is located differently.