What exactly is the second law of thermodynamics?

The second law of thermodynamics states that the amount of entropy (a measure of disorder) in the universe is increasing. So the universe tends to go from a more ordered state to a less ordered one. If you have a box which is half full of white sand and half full of black sand and shake it a lot, the sand gets all mixed up. But you can't shake the mixed up sand and have it reorganize itself. Any kind of order that can come about (like us living beings) has to come at the expense of creating a greater overall disorder. We eat food and turn ordered chemical energy into disordered thermal energy thereby producing lots of entropy and allowing us to organize are bodies and our surroundings (which very slightly decreases entropy) without violating the 2nd law overall.