Can someone explain to me the three laws of thermodynamics?

Prior to the enunciation of the first law of thermodynamics, scientists used to think that there are only two forms of energy, kinetic and potential. But when it was observed that they do not explian te enrgy conservation completely, it was realised that the energy goes inside the substances as thei inernal energy and not observable as motion of bulk matter. For example when a stone falls from a height on the earth where does it kinetic energy go? It gets distributted into the molecules of the stone and earth and some of it is used up in breaking the earth. So it was something like exension of law of conservation of mecahnical energy[kinetic + potenial] to include the ineternal energy of the substances locked in their atoms and molecules, sometimes losely called as heat because such an energy results in increase of temperature but not all times.

Second law relates to certain things never happenning although they do not involve violation of any so far known law of physics. For example, heat never flows from cold body to hot body. Although if it would have flowed it would have not violated energy conservation law but it does not happen. Also it was technologically noted that one cannot extract heat/internal energy from a substance and completely convert it into work. Although reverse is possible one can work and the whole of it could be converted into internal energy as for example when we rub our palms together, whatever work you do gets converted into your hands' internal energy. But now if you want to get it back from your hands into equivalent amount of work it would not be possible; some amount of this heat will have to be rejected at lower temperature. Work refers to some kind of ordered form of energy when it becomes part of internal energy it becomes randomised into the kinetic and potentail energy of the molecules at the microscopic level. So change is spontaneously favoured in the direction of increasing disorder. There are various ways of stating this law. In terms of energy it is stated as under:

Although energy is conserved it runs downhill, it all the time gets converted from more useful form to less useful form. You can also define it in terms of concept of entropy, in terms of efficiency of heat engines and refrigeration efficiency and so on.

As far as the third law it states that the entropy of a substance is zero at absolute temperature. This means classically there is perfect order at absolte zero and all motions stop. But we cannot reach this temperature in finite steps. So we can never reach that tempeature. Because if we really reach that temperature the second law will also be violated.

There is zeroeth law of thermodynamics which defines temperature formally not exactly in terms of heat as we know from our experiences. But as a condition which decides whteher the measurable properties of two sytems when brought into thermal contact will change or not change. If they change means their temperatures are different if they do not means there temperature are same. Whwnever we measure temperature of a substance we lok for some other property and see whether it s changing or not . Only when it does not cahnge we note down the temperature. It is also enunciated in terms of three systems anf their conditions of equilibrium,

Summarising, Zeroeth law defines temperature, first law defines internal energy, second law defines entropy and the third law talks about the non-attainability of absolute zero.

To understand Entropy see this link:

http://en.wikipedia.org/wiki/Entropy

As for the third law, it implies that reducing the temperature to absolute zero reduces the entropy to zero. It is believed to be impossible to reach this point, however magnetocaloric equipment has helped to reach very close.