Second Law of Thermodynamics
The second law of Thermodynamics (along with the Clausius' inequality) asserts the principle of increase of Entropy.
Putting it in simple words, Entropy of an isolated system cannot decrease : well it's always on an increase.
Put it the other way, the universe evolves in such a manner that the total entropy of the universe always increases.
The second law of thermodynamics, assigns directionality to natural processes.
Why does a fruit ripe ? What causes a spontaneous chemical reaction to take place ? Why do we age ?
All these processes happen because there is some increase of entropy associated with these. Whereas, reverse processes (such as we don't get younger) do not take place naturally.
All of these have a directionality associated with them.
Such a directionality of all natural processes is asserted by the second law in terms of entropy.
There are various statements associated with the second law of thermodynamics. All of them being logically equivalent. The most logical statement is the one involving entropy increase.
So, let me introduce the other equivalent statements of the same law.
Kelvin-Planck's statement -
No cyclic process is possible whose sole result is the complete conversion of heat into an equivalent amount of work.
Clausius' statement -
No cyclic process is possible whose sole effect would be the transfer of heat from a colder body to a hotter body.
All irreversible (natural and spontaneous) processes are characterized by the fact that entropy always increases in such processes.
And the second law of thermodynamics logically means that entropy always tends to increase.
A physical system shall always proceed to a state of maximum entropy.
In other words, second law specifies the direction of evolution of a natural process.
Natural systems always have a tendency to maximise their entropy.
And that's what the second law is all about.
Consider for example, the transfer of heat from one body to another in contact due to the temperature difference.
Heat always flows from a hotter body to a colder one spontaneously. But, no one has ever observed spontaneous transfer of heat from a colder body to a hotter body.
Even though such phenomenon are permissible by the first law, such processes never occur naturally. That's the essence of the second law.
Heat is transferred from a hotter body to a colder body because it is accompanied by entropy increase but, the converse never occurs because as such the entropy of the system is required to decrease.
That's what the Clausius' statement is all about.
It may be proved that all statements of the second law are completely equivalent and revolves around the same central concept of entropy increase.
It may be noted that, the transfer of heat from a colder body to a hotter body is possible (as in a refrigerator or an air conditioner). Second law states that such a process is however not spontaneous and natural. In order for such a process to take place, external work is required.