Since water and oil don't mix, when you dump water on a grease fire, it sinks right through to the bottom of the pan and evaporates instantly, spreading flaming oil everywhere. As you see in this video from The Slow Mo Guys, this could be a serious problem if it happens in your kitchen. Even recorded at 2500 frames per second, the explosion happens pretty fast.
There are a number of ways to properly extinguish a grease fire—using a metal lid to cut off the oxygen supply, dousing it with salt or baking soda, draping a wet, but not dripping, rag over it—but the most important thing to remember is just don't pour water on it.
[h/t Huffington Post]