The white stuff is calcium carbonate. Unless you have your own well, your municipal water supply is treated with calcium carbonate - it is called a "precipitate" because it attracts minute floating particles in the water, allowing them to sink to the bottom, leaving the water clear. In your water heater, small amounts remain and over months and years, the calcium settles at the bottom.
When you drain your water heater for winterizing, a small amount of water (maybe a quart) remains BELOW the level of the drain plug. If you use a turkey baster or siphon to drain water below the drain plug, you will extract a very milk like fluid; which is water mixed with calcium carbonate.
Now you know more than you wanted to know. Right?