1. Energy cannot be created or destroyed. This basically means that you cannot get something for nothing. As a result, the amount of energy in the entire universe is a constant value. All that we observe is the exchange of energy from one from to another. For example the energy from the sun is used to grow vegatables, we inturn eat these vegatables to gain energy. We then walk, run, work etc. and expend this energy which we really received from the sun!
2. The entropy of a system always increases Entropy is basically the randomness or order of a system. The entropy always increasing, means that the randomness of the system increases or the order of the system decreases. The system is becoming more chaotic. For example, a liquid has more entropy than a solid and less entropy than a gas. This is because a solid has a very definite structure and is well ordered. Whereas a liquid has less order and a gas has even less! See the figure below, which depicts the structure of a solid, liquid and gas respectively.
You might ask, well how can I freeze water if entropy is always suppose to increase? The answer to this is that you have to take energy out of the system in order to freeze the ice cube. You need to slow the water molecules down.
3. You can never reach absolute zero. This is just a way of saying that you can never decrease the temperature to a point where the system has no energy, meaning it does not move. The temperature where this would happend is known as absolute zero, and is -273.15 degrees Celcius! Otherwise known as 0K or zero degress Kelvin.
The following site contains an excellent summary of all areas of thermodynamics. Please see the HYPER PHYSICS SITE.