Laws of Thermodynamics

First Law - energy can be converted from one form to another but cannot be created or destroyed

Second Law - entropy is always going down and never going up

Entropy can be view as a measurement of ability to expend energy.  Lower entropy means higher ability.  For example, a charged battery have a higher entropy than a flat one.  The second law also defines how time flows, from lower entropy to higher entropy state.

Third Law - the entropy for a pure crystal is zero when it is at absolute zero temperature

Zeroth Law - when 2 bodies are with the same temperature with the third bodies, all three of them are in thermodynamic equilibrium