Question

define entropy in a simple way

Answer 1

It is essentially a measure of order or disorder.

Answer 2

There is a brief definition here:
http://simple.wikipedia.org/wiki/Entropy

Answer 3

In simple thermodynamic processes a system transfers heat from a hot source to a cold reservoir but you never see heat going from a cold source to a warmer reservoir. Why not? Energy can be conserved. There must be something that forbids this. What is it? The answer was sought and found to be a variable that defines a Thermodynamic state like Pressure, Temperature and density. It was Entropy! If in a Thermodynamic process the heat transferred at every instant divided by the temperature at that instant is summed from the beginning of the process to the end we find that in an isolated irreversible system this sum is either zero or positive. This gives rise to a statement of the Second Law of Thermodynamics that for an isolated system entropy never deceases. This has been interpreted as in an isolated system disorder never decreases, that is Entropy never decreases.

Unlike other state variables like pressure , or temperature it is not measured directly but calculated. As I said by summing the heat transferred in the system divided by the temperature at every instant of the process from beginning to end.