Phil Plait, writing at Bad Astronomy:
In a nutshell, Curiosity drilled into a rock to examine it chemically, and found evidence of clay minerals, which form in water. We’ve seen this before on Mars, but another important aspect of this is that it looks like the water that formed these minerals was neutral, not too strongly acidic or basic. Other places on Mars have chemistry that would be hostile to life as we know it, containing molecules like perchlorates that are very reactive with organic molecules; that it; destroying them. Curiosity also found the presence of sulfur, phosphorus, and carbon, all necessary ingredients to make life as we know it.
Just six elements make up the basic ingredients for life—CHNOPS—carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulhur. Watch the video below, excerpted from “Hunting the Elements,” to learn more.