What is entropy? Explain with example ?

It is a measure of disorderness of molecules
(or)
randomness of a system
It is a state function
Generally S_(g) > S_(l) > S_(s) 
Change in entropy q(rev) $\mathrm{\Delta S}=\frac{{\mathrm{q}}_{\left(\mathrm{rev}\right)}}{\mathrm{T}}$
For any isolated system
$∆$S_(total) = $∆$S_(system) + $∆$S_{(surroundings)}