Von Mises Stress

I've been hearing this term for quite a time now. Mostly while using Ansys.
Almost all the stress analysis simulations I've gone through had this stress plot as a solution.

So What is this "Von Mises Stress " ?
      Why is it so useful ?

The concept of Von Mises Stress arises from the Distortion Energy failure theory. AKA

And this expression over here is our 
von mises stress

Most of the other Theories of Failure like maximum principle stress theory ,maximum shear stress theory, etc failed at many occasions but this Distortion Energy failure Theory holds up pretty well and hence is the most preferred failure theory in the Industry.

So while analyzing a machine, von mises stress plot is a sure shot way of determining whether the machine/part will  survive or not. 

If the maximum von mises stress is less than the yield point* value of the material used,  you are good to proceed.

*(of course you have to include factor of safety into the equation)