The Three Laws of Thermodynamics are the basic foundation of our understanding of how the Universe works. Everything, no matter how large or small, is subject to the Three Laws: stars, birds, human beings, dust motes in sunbeams, lobsters, you name it, the Three Laws cover it. Since the Laws are so obviously important, what are they?
First Law: Energy Can Be neither Created nor Destroyed
What is energy? No one's really sure, but there are two definitions.
1) Energy is whatever it is that makes up the Universe. Everything that exists, according to Albert Einstein, is some form of energy, although there are many different types. Whatever energy is, we can't seem to make any more of it or get rid of it; it can change to a different form, but the amount of it can't change.
2) Energy is also used to mean the potential for work that something has; the greater the ability of something to change the things around it, the more energy it has.
Second Law: The Entropy of an Isolated System always Increases
Entropy is a measure of how disorderly something is. A male student's dorm room will probably have a great deal of entropy. If he cleaned his room, the entropy would decrease.
Entropy is also a measure of how probable a configuration of energy is. The less likely a configuration is, the less entropy it has. Things that happen spontaneously have a very high entropy. Things that don't happen on their own have a much lower entropy.
Some people also use the word entropy to describe the process of energy spreading out into a more likely configuration.
Why does it matter how messed-up something is? It determines how much energy (definition 2) is in the Universe. In order to be able to change something, energy must move from a highly concentrated state to a less concentrated state. In other words, the energy has to spread out to do work. If the amount of order (or disorder) in two systems is the same, they can't do any work on each other.
Heat energy is an excellent example of this idea1. If you put a cold object next to a hot one, heat energy will flow out of the warmer object and into the cold one. Once the two objects have the same amount of heat energy, the flow stops.
You can also imagine two large empty tanks connected at the bottom by a small pipe. If you fill one of the tanks with water and open the pipe, water will begin to gush through the pipe into the second tank. As long as there is more water in one tank than in the other, you can make the water do work for you. You could make this moving water turn a paddlewheel and generate electricity, for example. As the water levels in the two tanks comes closer together, the water will flow more and more slowly through the pipe. Eventually, when there's the same amount of water in both tanks, the flow will stop and the water can't do any more work.
The more orderly something is, the more energy it has stored in it compared to something that's disorderly. So, if you want to do something, you can transfer the energy from one thing to another. The problem is that some of the energy you use will spontaneously turn into a less concentrated form than it was before. Usually, the wasted energy turns into heat. You experience this every day in the form of friction: two objects that are touching together as they move will generate heat. Every time energy changes form, some of it changes into heat. That means that when you're making something more orderly by messing something else up, the order you create will always be smaller than the disorder you make. Therefore, the amount of useful energy in the Universe is constantly reduced. If you want to store energy in something or make it more orderly, you have to cause a greater amount of energy to be released as something else is messed up, and some of the energy you lose will be lost forever.
The Second Law has dramatic and far-reaching consequences. It says that stars will eventually exhaust their fuel and go out, and there will come a time when no new stars will be born, because all of the Universe's energy will be spread too thin. On a more mundane level, the process of entropy is why things break down, burn out, and die. Some scientists (and science fiction writers) believe that entropy is the 'arrow of time'; the principle that makes time appear to move in only one direction and some things to happen in only one way. The Second Law is even responsible for death itself: just look what it did to Humpty Dumpty.
Humpty Dumpty sat on a wall,
Humpty Dumpty had a great fall.
All the King's horses and all the King's men
Couldn't put Humpty together again.
The Second Law is responsible not only for the eventual destruction of the Universe, but for the mortality of every human being as well. That, ladies and gentlemen, is entropy.
Third Law: The Entropy of a Perfect Crystal at Absolute Zero is Zero
This is just a precise definition of the idea of entropy. A perfectly orderly arrangement of atoms that didn't have any vibrations because of heat energy would have no entropy at all. It's also infinitely improbable for such a thing to happen, as absolute zero is impossible to reach, as far as we know. This Law states that everything in the Universe will always be just a little bit messy and imperfect.